FileSets: Don't crash if a file's file set isn't found

File sets currently have issues with files containing semicolons;
particularly, names get mangled differently in different contexts. This
can result in failing to find a file's file set. Prior to dee8799d16
(FileSet management: Introduce dedicated classes for generation,
2026-02-16, v4.4.0-rc1~691^2), we guarded against this, but the
aforementioned commit introduced code that would try to use the found
fileset object without checking if it was valid. Add the missing checks
so that we do not crash trying to dereference a null pointer. Also,
refactor `cmGeneratorTarget::GetSourceFilePaths` to avoid unnecessary
work.

Fixes: #27924