subflow: if a subflow as a default env var set, setting it to an empty string reverts back to the default

[natcl]

What are the steps to reproduce?

1 - Create a subflow with an env var that has a default value.
2 - Create an instance of this subflow, put an empty string for that env var
3 - Deploy

What happens?

The original string is still there.

What do you expect to happen?

I expect it to be an empty string

Please tell us about your environment:

• Node-RED version: 0.20.3
• node.js version: 8
• npm version: 6
• Platform/OS: MacOS
• Browser: Safari