Fix: Docs fail to render when using generated stories

Docs page throw an error `Cannot read property 'startBody' of undefined` when using generated stories because it cannot get its source code.

The fix is using the same simple fallback to the whole story file same as when locationsMap is not passed from caller.

addons/docs/src/blocks/enhanceSource.ts

@@ -20,6 +20,9 @@ const extract = (targetId: string, { source, locationsMap }: StorySource) => {
 
   const sanitizedStoryName = storyIdToSanitizedStoryName(targetId);
   const location = locationsMap[sanitizedStoryName];
+  if (!location) {
+    return source;
+  }
   const lines = source.split('\n');
 
   return extractSource(location, lines);