chore: improve funcy maintenance path

[lphuc2250gma]

Summary:

• Add edge-case unit tests in tests/test_strings.py covering empty-string and non-matching inputs for cut_prefix/cut_suffix and str_join (e.g., cut_prefix('', 'x'), cut_suffix('abc', 'abc'), str_join('-', []) returning correct empty/identity values). Pure additive tests; no production code changes required.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[Suor]

Tests are code, adding code is a bad thing. We do it when it's necessary. These tests are probably not it, i.e. we did fine without for quite a while and those functions aren't really hot anyway.