[BUG] NPE when minBound/maxBound is not set before being called

[GeorgeAp]

Describe the bug
A NullPointerException occurs when the methods minBound() and maxBound

OpenSearch/server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/HistogramAggregationBuilder.java

Lines 228 to 236 in 8844f05

	/** Get the current minimum bound that is set on this builder. */
	public double minBound() {
	return extendedBounds.getMin();
	}
	/** Get the current maximum bound that is set on this builder. */
	public double maxBound() {
	return extendedBounds.getMax();
	}

are called before they are set in DoubleBounds

OpenSearch/server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/DoubleBounds.java

Lines 154 to 160 in 8844f05

	public Double getMin() {
	return min;
	}
	public Double getMax() {
	return max;
	}

To Reproduce
A unit test that does not set the min and max values, example for the minBound:

public void testNPE() {
        HistogramAggregationBuilder factory = new HistogramAggregationBuilder("foo");

        try {
            factory.minBound();
        }
        catch (Exception e) {
            assertThat(e, instanceOf(NullPointerException.class));
        }
    }

Expected behavior
It should return null

Host/Environment (please complete the following information):

• Java version : 11
• OS: Ubuntu
• Version 20.04.4 LTS