Frontend compilation fails if TS files are only in the fallback bundle

[Artur-]

Description of the bug

In Vaadin 14, tsconfig.json is only generated if the project contains .ts files. However, if the TS files end up in the fallback bundle, for instance because of #14732, then no tsconfig.json is generated. The build then fails with errors like

ERROR in [tsl] ERROR
      TS18002: The 'files' list in config file 'tsconfig.json' is empty.
 @ ../target/frontend/generated-flow-imports-fallback.js 21:0-46
 @ ../target/frontend/generated-flow-imports.js?babel-target=es5

I can imagine this would happen also if you add an addon with TS files and you do not use the addon but I have not tested that.

Expected behavior

Compilation works when TS files are included only in the fallback bundle

Minimal reproducible example

https://github.com/samie/specialbuttons/commit/571832b121b86653f87c02740a0d3e7e6134eede#diff-9c5fb3d1b7e3b0f54bc5c4182965c4fe1f9023d449017cece3005d3f90e8e4d8

Versions

• Vaadin / Flow version: 14.8.18