create string file if this not exist

gradle/scripts/stringGenerator.gradle

@@ -32,6 +32,11 @@ private def generate(Map<String, String> sources, Project project) {
         }
 
         def stringsFile = getFile(key, key == defaultLang, project)
+        if (!stringsFile.exists()) {
+            stringsFile.parentFile.mkdirs()
+            stringsFile.createNewFile()
+        }
+
         stringsFile.write(sw.toString(), "UTF-8")
     }
 }