Make parse_run_config_input more defensive (#7003)

Summary:
Right now if you pass in empty string for runConfigData this codepath will fail with an error. Coerce it to {} instead.

Test Plan: BK

python_modules/dagster-graphql/dagster_graphql/schema/runs.py

@@ -144,7 +144,7 @@ class Meta:
 
 
 def parse_run_config_input(run_config):
-    return json.loads(run_config) if isinstance(run_config, str) else run_config
+    return json.loads(run_config) if (run_config and isinstance(run_config, str)) else run_config
 
 
 types = [

python_modules/dagster-graphql/dagster_graphql_tests/graphql/test_config_types.py

@@ -155,6 +155,19 @@ def test_basic_valid_config_serialized_config(self, graphql_context):
         assert result.data["isPipelineConfigValid"]["__typename"] == "PipelineConfigValidationValid"
         assert result.data["isPipelineConfigValid"]["pipelineName"] == "csv_hello_world"
 
+    def test_basic_valid_config_empty_string_config(self, graphql_context):
+        result = execute_config_graphql(
+            graphql_context,
+            pipeline_name="csv_hello_world",
+            run_config="",
+            mode="default",
+        )
+
+        assert not result.errors
+        assert result.data
+        assert result.data["isPipelineConfigValid"]["__typename"] == "RunConfigValidationInvalid"
+        assert result.data["isPipelineConfigValid"]["pipelineName"] == "csv_hello_world"
+
     def test_root_field_not_defined(self, graphql_context):
         result = execute_config_graphql(
             graphql_context,