autocomplete [nfc]: Add [MentionAutocompleteView.query] getter

When adding this, I got a diagnostic error
`getter_not_subtype_setter_types`:
  https://dart.dev/tools/diagnostic-messages#getter_not_subtype_setter_types

Shrug; I guess it's fine to adjust the setter param's type to the
higher type, with code to handle the difference. Discussion:
  zulip#241 (comment)

Author: chrisbobbe

lib/model/autocomplete.dart

@@ -192,10 +192,13 @@ class MentionAutocompleteView extends ChangeNotifier {
   final PerAccountStore store;
   final Narrow narrow;
 
+  MentionAutocompleteQuery? get query => _query;
   MentionAutocompleteQuery? _query;
-  set query(MentionAutocompleteQuery query) {
+  set query(MentionAutocompleteQuery? query) {
     _query = query;
-    _startSearch(query);
+    if (query != null) {
+      _startSearch(query);
+    }
   }
 
   /// Recompute user results for the current query, if any.