Allow selective caching for `--fix` and `--diff` (#8316)
## Summary
If a file has no diagnostics, then we can read and write that
information from and to the cache, even if the fix mode is `--fix` or
`--diff`. (Typically, we can't read or write such results from or to the
cache, because `--fix` and `--diff` have side effects that take place
during diagnostic analysis (writing to disk or outputting the diff).)
This greatly improves performance when running `--fix` on a codebase in
the common case (few diagnostics).
Closes #8311.
Closes https://github.com/astral-sh/ruff/issues/8315.
