how kie handle empty lable value depends on storage imple

[tianxiaoliang]

Describe the bug A clear and concise description of what the bug is.

To Reproduce Steps to reproduce the behavior:

1. set a key value with lable "app=default, env="

2. use etcd as storage

3. query by "app=default, env="

4. it will not return any result

5. set a key value with lable "app=default, env="

6. use mongo as storage

7. query by "app=default, env="

8. it will not return result

Expected behavior return same result

Sugesstion for bug:

change service layer， if lable value is empty, do not pass it to dao layer, consider it as an inefficiency query param