fix(GraphQL): This PR fix panic error when we give null value in filt…

…er connectives. (#6707)

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
    },

```

graphql/resolve/query_rewriter.go

@@ -1020,6 +1020,9 @@ 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{}
@@ -1076,7 +1079,6 @@ func buildFilter(typ schema.Type, filter map[string]interface{}) *gql.FilterTree
 					for _, v := range vals {
 						args = append(args, gql.Arg{Value: maybeQuoteArg(fn, v)})
 					}
-
 				case "near":
 					//  For Geo type we have `near` filter which is written as follows:
 					// { near: { distance: 33.33, coordinate: { latitude: 11.11, longitude: 22.22 } } }
@@ -1086,19 +1088,18 @@ func buildFilter(typ schema.Type, filter map[string]interface{}) *gql.FilterTree
 					coordinate, _ := geoParams["coordinate"].(map[string]interface{})
 					lat := coordinate["latitude"]
 					long := coordinate["longitude"]
+					args = append(args, gql.Arg{Value: fmt.Sprintf("[%v,%v]", long, lat)},
+						gql.Arg{Value: fmt.Sprintf("%v", distance)})
 
-          args = append(args, gql.Arg{Value: fmt.Sprintf("[%v,%v]", long, lat)})
-          args = append(args, gql.Arg{Value: fmt.Sprintf("%v", distance)})
-
-        default:
+				default:
 					args = append(args, gql.Arg{Value: maybeQuoteArg(fn, val)})
 				}
 				ands = append(ands, &gql.FilterTree{
 					Func: &gql.Function{
 						Name: fn,
 						Args: args,
 					},
-				})  
+				})
 			case []interface{}:
 				// ids: [ 0x123, 0x124 ] -> uid(0x123, 0x124)
 				ids := convertIDs(dgFunc)

graphql/resolve/query_test.yaml

@@ -15,7 +15,7 @@
         dgraph.uid : uid
       }
     }
-    
+
 -
   name: "Geo query"
   gqlquery: |
@@ -206,6 +206,22 @@
       }
     }
 
+-
+  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: "Query with has Filter"
   gqlquery: |