Deleting the content of the npmrc setting does not revert back to template value

### Current Behavior

- add npmrc value to template (unlocked)
- start instance with template
- edit instance npmrc file to new value
- later want default template value back, delete all content from npmrc field

The value is now empty and no way to signal you want the template value back

### Expected Behavior

Tricky, not sure about this one. 2 options

1. current behaviour is correct as removing the content is a valid state
2. if empty should revert to template value

### Steps To Reproduce

See current behaviour

### Environment

- FlowFuse version: 2.7.0
- Node.js version: 18.x
- npm version: 9.x
- Platform/OS: All
- Browser: All

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate