search: return null on whitespace search term

lib/search/word_search/entry_id_query.dart

@@ -246,12 +246,12 @@ Future<List<ScoredEntryId>> fetchEntryIds(
   int? pageSize,
   int? offset,
 ) async {
+  assert(word.trim().isNotEmpty, 'Word should not be empty when fetching entry IDs');
+
   if (searchMode == SearchMode.auto) {
     searchMode = _determineSearchMode(word);
   }
 
-  assert(word.isNotEmpty, 'Word should not be empty when fetching entry IDs');
-
   late final List<ScoredEntryId> entryIds;
   switch (searchMode) {
     case SearchMode.kanji:

lib/search/word_search/word_search.dart

@@ -41,7 +41,7 @@ Future<List<WordSearchResult>?> searchWordWithDbConnection(
   int page = 0,
   int? pageSize,
 }) async {
-  if (word.isEmpty) {
+  if (word.trim().isEmpty) {
     return null;
   }