Fix char at error on undefined when opening websocket-listner

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Opening websocket-listner settings panel causes following editor error:

Definition error: websocket-listener.label TypeError: Cannot read property 'charAt' of undefined
    at Object.label (<anonymous>:113:27)
    at Object.getNodeLabel (red.min.js:16)
    at Object.p [as refresh] (red.min.js:16)
    at Object.show (red.min.js:16)
    at red.min.js:16

This PR tries to fix this problem.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.02%) to 77.31% when pulling 7694349 on node-red-hitachi:fix-charAt-error-on-undefined into 81a4f42 on node-red:dev.