search: return null on whitespace search term

lib/search/word_search/entry_id_query.dart

@@ -246,12 +246,15 @@ 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;
   }
 

test/search/word_search_test.dart

@@ -4,6 +4,22 @@ import 'package:test/test.dart';
 import 'setup_database_connection.dart';
 
 void main() {
+  test('Search empty string - auto', () async {
+    final connection = await setupDatabaseConnection();
+    final result = await connection.jadbSearchWord('');
+    expect(result, isNull);
+  });
+
+  test('Search whitespace - auto', () async {
+    final connection = await setupDatabaseConnection();
+    final result1 = await connection.jadbSearchWord('   ');
+    expect(result1, isNull);
+    final result2 = await connection.jadbSearchWord('\t');
+    expect(result2, isNull);
+    final result3 = await connection.jadbSearchWord('\n');
+    expect(result3, isNull);
+  });
+
   test('Search a word  - english - auto', () async {
     final connection = await setupDatabaseConnection();
     final result = await connection.jadbSearchWord('kana');