Support default store path shortcut for isOpen.

Author: greenrobot-team

objectbox/lib/src/native/store.dart

@@ -267,10 +267,11 @@ class Store {
     }
   }
 
-  /// Returns if an open store was found for the given directory
-  /// (i.e. opened before and not yet closed).
-  static bool isOpen(String directory) {
-    final cStr = directory.toNativeUtf8();
+  /// Returns if an open store (i.e. opened before and not yet closed) was found
+  /// for the given [databasePath] (or if null the [defaultDatabasePath]).
+  static bool isOpen(String? databasePath) {
+    final path = _safeDatabasePath(databasePath);
+    final cStr = path.toNativeUtf8();
     try {
       return C.store_is_open(cStr.cast());
     } finally {