Do key validation only when key is provided

key_id is optional, the previous code didn't handle the case of key_id
not specified

packaging/apt_key

@@ -188,6 +188,7 @@ def main():
     state           = module.params['state']
     changed         = False
 
+    if key_id:
         try:
             _ = int(key_id, 16)
             key_id = key_id.lstrip('0x')