feat(developer): kmc convert typesafety 😎

[SabineSIL]

In several places in kmc-convert: keylayout->kmn we did not care about return type null or undefined for function return values or parameters. Therefore we got several warnings about possible return types not being assignable to symbols.

Even though they did not affect the code running correctly they have been specified more clearly in this PR.

@keymanapp-test-bot skip

see also:

• feat(developer): kmc-convert: new conversion .keylayout->kmn 😎 #12564

[keymanapp-test-bot]

User Test Results

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Test Artifacts

• Developer
  • Keyman Developer - build : all tests passed (no artifacts on BuildLevel "build")
  • Compiler Regression Tests - build : all tests passed (no artifacts on BuildLevel "build")
  • Keyman Developer (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")
  • kmcomp.zip - build : all tests passed (no artifacts on BuildLevel "build")
  • kmcomp.zip (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")
• Keyboards
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