gh-159693: Format import errors with no usable search paths

[limadog9]

Fixes #159693.

When sys.path has no existing directories, the extension-module hint constructs an empty MultiplexedPath and raises FileNotFoundError while formatting the original ModuleNotFoundError.

This also affects the REPL: InteractiveColoredConsole.runcode() calls traceback.format_exception() through its exception hook. I reproduced the secondary exception through that console before the fix and verified that it displays the original import error afterward.

Return without a hint when there are no directories to search. The regression covers both an empty sys.path and a path containing only a nonexistent directory.

Validation on a Linux x86-64 debug build:

• Both regression cases fail on unmodified main and pass with the fix.
• ./python -m test -j2 test_traceback test_code test_pyrepl passed (833 test cases, 65 skips).
• Ruff, NEWS reStructuredText lint, patchcheck, and git diff --check passed.

• Issue: traceback formatting raises FileNotFoundError with empty sys.path #159693