[Bug libstdc++/101203] Remove unnecessary empty check in std::function

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101203

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
PR 56551 uses a similar idea. It wouldn't be ABI compatible with existing code
though.