adj: do not generate assets.py when don't have resource

dev_tools/button_extract.py

@@ -218,12 +218,15 @@ class ModuleExtractor(ModuleExtractor):
             v.sort()
 
         logger.info('Module: %s(%s)' % (self.name, len(exp)))
-        exp = IMPORT_EXP + exp
+        if exp:
+            exp = IMPORT_EXP + exp
         for prefix_name in prefix_assets.keys():
             prefix_assets[prefix_name] = IMPORT_EXP + prefix_assets[prefix_name]
         return exp, prefix_assets
 
     def write_to_file(self, file, text):
+        if not text:
+            return
         with open(file, 'w', newline='') as f:
             for line in text:
                 f.write(line + '\n')