bug(common,developer): improve null handling and initialisation of KvkFileWriter

### Describe the bug

There are problems with null handling of `associatedKeyboard`, key `ansiFont.name` and `unicodeFont.name` in `KvkFileWriter`, plus incorrect initialisation of `BUILDER_KVK_STRING`, which should be `{ len: 1, str: '' }` not `{ len: 0, str: '' }`.

Test cases have been written in PR #12734 to cover the three null strings, but are commented out as they fail.

### Keyman apps

- [ ] Keyman for Android
- [ ] Keyman for iPhone and iPad
- [ ] Keyman for Linux
- [ ] Keyman for macOS
- [ ] Keyman for Windows
- [ ] Keyman Developer
- [x] KeymanWeb
- [ ] Other - give details at bottom of form