Don't crash when fn2ftp is called with nullptr

std::filesystem strikes again.

Adding a safe overload to handle such cases since there are multiple places in the code where we pass dubious (not necessarily null, but I cannot analyze them all) char* to this function.

This can also help refactor away such calls in the main branch by doing a code search for the new overload instead of checking all calls to fn2ftp.

Fixes #4756 (closed)