[flutter_tools] Handle null version gracefully in CachedArtifact and MaterialFonts (#191494)

When artifact version files (such as
`bin/internal/material_fonts.version`) are missing or cannot be
resolved, `CachedArtifact.version` evaluates to `null`. Previously,
`CachedArtifact.update` invoked `updateInner` before checking if
`version` was `null`, causing `MaterialFonts.updateInner` (and other
artifact subclasses) to evaluate `version!` and throw an unhandled
`TypeError` / `_CastError` with `Null check operator used on a null
value`.

This change:
1. Returns early with a warning in `CachedArtifact.update` before
attempting `updateInner` or writing stamps when `version == null`.
2. Guards against `version == null` in `MaterialFonts`, `GradleWrapper`,
and `FlutterRunnerDebugSymbols`.
3. Adds an automated regression test in `cache_test.dart`.

Fixes #90092