DatabasePlugin: add FLAG_REQUIRE_STORAGE

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

src/storage/Configured.cxx

@@ -76,3 +76,9 @@ CreateConfiguredStorage(Error &error)
 
 	return CreateConfiguredStorageLocal(error);
 }
+
+bool
+IsStorageConfigured()
+{
+	return config_get_string(CONF_MUSIC_DIR, nullptr) != nullptr;
+}