Query: Avoid PHP notices when get_queried_object() returns null.

`WP_Query` methods assume that `get_queried_object()` would return a non-null value, which is not always the case.

This commit resolves various warnings in `WP_Query` along the lines of:
{{{
Attempt to read property "post_type" on null in wp-includes/class-wp-query.php on line 4338
}}}

Follow-up to [1728], [3639], [8807], [49119].

Props dd32, yellyc, boonebgorges, darkskipper, Howdy_McGee, swissspidy, nacin, mikeschroder, mikejolley, sterlo, datainterlock, utsavmadaan823, kanlukasz, woji29911, hellofromTonya, zikubd, deksar, bwbama, noplanman, nouarah, SergeyBiryukov.
Fixes #29660.

git-svn-id: https://develop.svn.wordpress.org/trunk@54496 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-query.php

@@ -3963,6 +3963,9 @@ public function is_attachment( $attachment = '' ) {
 		$attachment = array_map( 'strval', (array) $attachment );
 
 		$post_obj = $this->get_queried_object();
+		if ( ! $post_obj ) {
+			return false;
+		}
 
 		if ( in_array( (string) $post_obj->ID, $attachment, true ) ) {
 			return true;
@@ -3996,6 +3999,9 @@ public function is_author( $author = '' ) {
 		}
 
 		$author_obj = $this->get_queried_object();
+		if ( ! $author_obj ) {
+			return false;
+		}
 
 		$author = array_map( 'strval', (array) $author );
 
@@ -4032,6 +4038,9 @@ public function is_category( $category = '' ) {
 		}
 
 		$cat_obj = $this->get_queried_object();
+		if ( ! $cat_obj ) {
+			return false;
+		}
 
 		$category = array_map( 'strval', (array) $category );
 
@@ -4068,6 +4077,9 @@ public function is_tag( $tag = '' ) {
 		}
 
 		$tag_obj = $this->get_queried_object();
+		if ( ! $tag_obj ) {
+			return false;
+		}
 
 		$tag = array_map( 'strval', (array) $tag );
 
@@ -4315,6 +4327,9 @@ public function is_page( $page = '' ) {
 		}
 
 		$page_obj = $this->get_queried_object();
+		if ( ! $page_obj ) {
+			return false;
+		}
 
 		$page = array_map( 'strval', (array) $page );
 
@@ -4422,6 +4437,9 @@ public function is_single( $post = '' ) {
 		}
 
 		$post_obj = $this->get_queried_object();
+		if ( ! $post_obj ) {
+			return false;
+		}
 
 		$post = array_map( 'strval', (array) $post );
 
@@ -4469,6 +4487,9 @@ public function is_singular( $post_types = '' ) {
 		}
 
 		$post_obj = $this->get_queried_object();
+		if ( ! $post_obj ) {
+			return false;
+		}
 
 		return in_array( $post_obj->post_type, (array) $post_types, true );
 	}

tests/phpunit/tests/query.php

@@ -788,4 +788,113 @@ public function test_is_post_type_archive_should_return_false_for_an_undefined_p
 
 		$this->assertFalse( is_post_type_archive( $post_type ) );
 	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_singular_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'pagename' => 'non-existent-page',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+		$this->assertFalse( $q->is_single() );
+
+		$this->assertTrue( $q->is_singular() );
+		$this->assertFalse( $q->is_singular( 'page' ) );
+
+		$this->assertTrue( $q->is_page() );
+		$this->assertFalse( $q->is_page( 'non-existent-page' ) );
+	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_single_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'name' => 'non-existent-post',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+		$this->assertFalse( $q->is_page() );
+
+		$this->assertTrue( $q->is_singular() );
+		$this->assertFalse( $q->is_singular( 'post' ) );
+
+		$this->assertTrue( $q->is_single() );
+		$this->assertFalse( $q->is_single( 'non-existent-post' ) );
+	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_attachment_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'attachment' => 'non-existent-attachment',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+
+		$this->assertTrue( $q->is_singular() );
+		$this->assertFalse( $q->is_singular( 'attachment' ) );
+
+		$this->assertTrue( $q->is_attachment() );
+		$this->assertFalse( $q->is_attachment( 'non-existent-attachment' ) );
+	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_author_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'author_name' => 'non-existent-author',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+
+		$this->assertTrue( $q->is_author() );
+		$this->assertFalse( $q->is_author( 'non-existent-author' ) );
+	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_category_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'category_name' => 'non-existent-category',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+
+		$this->assertTrue( $q->is_category() );
+		$this->assertFalse( $q->is_tax() );
+		$this->assertFalse( $q->is_category( 'non-existent-category' ) );
+	}
+
+	/**
+	 * @ticket 29660
+	 */
+	public function test_query_tag_404_does_not_throw_warning() {
+		$q = new WP_Query(
+			array(
+				'tag' => 'non-existent-tag',
+			)
+		);
+
+		$this->assertSame( 0, $q->post_count );
+
+		$this->assertTrue( $q->is_tag() );
+		$this->assertFalse( $q->is_tax() );
+		$this->assertFalse( $q->is_tag( 'non-existent-tag' ) );
+	}
 }