Change Value Used in DB Plugin

Summary:
- Add value for null type to use `.value` easier
- Add value to string by directly converting to string if possible; otherwise null

Reviewed By: jknoxville

Differential Revision: D21788242

fbshipit-source-id: f3a9f995de6b4cb1b304981c8adaaba70105c988

desktop/app/src/index.tsx

@@ -91,7 +91,11 @@ export {
   default as ManagedTable_immutable,
   ManagedTableProps_immutable,
 } from './ui/components/table/ManagedTable_immutable';
-export {Value, renderValue} from './ui/components/table/TypeBasedValueRenderer';
+export {
+  Value,
+  renderValue,
+  valueToNullableString,
+} from './ui/components/table/TypeBasedValueRenderer';
 export {
   DataValueExtractor,
   DataInspectorExpanded,