Fix searchDiscover handling invalid results (#1013)

* Filter out bad discover search results

An stub object is sometimes returned from the API that is marked as
`'external'` but has has no `'SearchResults'` key.

    {'id': 'external', 'title': 'More Ways To Watch', 'size': 0}

* Wrap line to fit 127c

* Ensure empty array for SearchResult

Author: josh

plexapi/myplex.py

@@ -899,7 +899,7 @@ def searchDiscover(self, query, limit=30, libtype=None):
 
         data = self.query(f'{self.METADATA}/library/search', headers=headers, params=params)
         searchResults = data['MediaContainer'].get('SearchResults', [])
-        searchResult = next((s['SearchResult'] for s in searchResults if s.get('id') == 'external'), [])
+        searchResult = next((s.get('SearchResult', []) for s in searchResults if s.get('id') == 'external'), [])
 
         results = []
         for result in searchResult: