fix(developer): compiler emitting garbage for readonly groups 🎾 🍒

[mcdurdin]

Picked up while reviewing code for conversion to C++. The compiler could, in some circumstances emit garbage code for readonly groups because we weren't testing all scenarios correctly. In practice, this would have been rare as readonly groups don't emit characters, but it should be fixed!

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• Windows
  • Keyman for Windows
  • FirstVoices Keyboards for Windows

[ermshiperete]

By the way, :cherries:-picked from #7013 :smile:

[keyman-server]

Changes in this pull request will be available for download in Keyman version 15.0.269