Fix(Indexing): Improve error reporting for missing data during vector…

… searches (#60)

Description: If the vector parameter in a similar_to function call is a
variable that evaluates to NULL then we report it as a parsing (syntax)
error.

The fix, instead of reporting a parsing error,  returns an empty result.

Closes: HYP-302

---------

Co-authored-by: Bill Province <billprovince@gmail.com>

graphql/schema/testdata/schemagen/input/embedding-directive-with-similar-queries.graphql

@@ -0,0 +1,21 @@
+# simple user product GraphQL API - v0.4
+
+type Product {
+  id: String! @id
+  description: String
+  title: String
+  imageUrl: String
+  product_vector: [Float] @hm_embedding 
+}
+
+type Purchase @lambdaOnMutate(add: true){
+  user: User @hasInverse(field: "purchase_history")
+  product: Product
+  date: DateTime @search(by: [day])
+}
+
+type User  {
+  email: String!  @id
+  purchase_history: [Purchase] 
+  user_vector: [Float] @hm_embedding 
+}