Provide fallback value for schema_only parameter in update_index management command

CONTRIBUTORS.rst

@@ -200,6 +200,7 @@ Contributors
 * Andreas Nüßlein
 * John Heasly
 * Nikolai Røed Kristiansen
+* Alex Zagorodniuk
 
 Translators
 ===========

wagtail/wagtailsearch/management/commands/update_index.py

@@ -117,7 +117,7 @@ class Command(BaseCommand):
 
         # Update backends
         for backend_name in backend_names:
-            self.update_backend(backend_name, schema_only=options['schema_only'])
+            self.update_backend(backend_name, schema_only=options.get('schema_only', False))
 
     def print_newline(self):
         self.stdout.write('')