Change setting.json file and reset checkstyle has an exception

[EricZhangzf]

Environment:

1. OS : Windows10
2. VS Code Version: 1.38.1
3. Extension Version: 1.3.0
4. Language Support for Java version: 0.49.0

Repro steps:

1. Open to-do project
2. Modify setting.json(workspace) file as below: { "java.checkstyle.configuration:" }
3. F1 -> Set the Checkstyle Configuration File -> select one

Result:

Json file no change.