backport: Require non-empty directive locations

Replicates graphql/graphql-js@993d7ce

Author: Cito

src/graphql/type/validate.py

@@ -154,6 +154,12 @@ def validate_directives(self) -> None:
             # Ensure they are named correctly.
             self.validate_name(directive)
 
+            if not directive.locations:
+                self.report_error(
+                    f"Directive @{directive.name} must include 1 or more locations.",
+                    directive.ast_node,
+                )
+
             # Ensure the arguments are valid.
             for arg_name, arg in directive.args.items():
                 # Ensure they are named correctly.

tests/type/test_validation.py

@@ -446,6 +446,21 @@ def rejects_a_schema_whose_directives_are_incorrectly_typed():
             {"message": "Expected directive but got: SomeScalar."},
         ]
 
+    def rejects_a_schema_whose_directives_have_empty_locations():
+        bad_directive = GraphQLDirective(
+            name="BadDirective1",
+            locations=[],
+        )
+        schema = GraphQLSchema(
+            SomeObjectType,
+            directives=[bad_directive],
+        )
+        assert validate_schema(schema) == [
+            {
+                "message": "Directive @BadDirective1 must include 1 or more locations.",
+            },
+        ]
+
 
 def describe_type_system_objects_must_have_fields():
     def accepts_an_object_type_with_fields_object():