fix(common/web): improve null handling and initialisation of KvkFileWriter

[markcsinclair]

Improve 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 included to cover the three null strings.

Fixes: #12809

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

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Test Artifacts

• Android
  • Keyman for Android apk
  • FirstVoices for Android 16.0 apk
  • FirstVoices Keyboards for Android apk
  • KeyboardHarness apk
  • Keyman for Android 16.0 apk
  • KMSample1 apk
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• Developer
  • Keyman Developer - build pending
  • Compiler Regression Tests - build pending
  • kmcomp.zip - build pending
• iOS
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  • TestFlight internal PR build version - 18.0.167 (0.12810.12531)
• Keyboards
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• Web
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• Windows
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