fix(GraphQL): This PR fix panic error when we give null value in filter connectives. (#6707) (#6736)

Fixes GRAPHQL-744

This PR fix panic error when we give null value in filter connectives. Null values corresponding to connective are skipped now.
For example in the below filter, not connective gets skipped.

```

 filter:{
      id:{eq:"123"},
      not:null
    },

```

(cherry picked from commit 345b6d1)

Author: JatinDev543

graphql/resolve/query_rewriter.go

@@ -1014,6 +1014,9 @@ func buildFilter(typ schema.Type, filter map[string]interface{}) *gql.FilterTree
 	// Each key in filter is either "and", "or", "not" or the field name it
 	// applies to such as "title" in: `title: { anyofterms: "GraphQL" }``
 	for _, field := range keys {
+		if filter[field] == nil {
+			continue
+		}
 		switch field {
 
 		// In 'and', 'or' and 'not' cases, filter[field] must be a map[string]interface{}

graphql/resolve/query_test.yaml

@@ -167,6 +167,21 @@
       }
     }
 
+-
+  name: "Filter connectives with null values gets skipped "
+  gqlquery: |
+    query {
+      queryAuthor(filter: { name: { eq: "A. N. Author" },not:null }) {
+        name
+      }
+    }
+  dgquery: |-
+    query {
+      queryAuthor(func: type(Author)) @filter(eq(Author.name, "A. N. Author")) {
+        name : Author.name
+        dgraph.uid : uid
+      }
+    }
 -
   name: "Filters in same input object implies AND"
   gqlquery: |