file(RPATH): Restore tolerance of unknown formats if new RPATH is empty

Since commit 2e114987 (cmSystemTools: Support multiple binary formats,
2021-06-14, v3.22.0-rc1~575^2) the `file(RPATH_...)` operations fail on
files that are not ELF or XCOFF format.  Previously the RPATH operations
tolerated files of unknown format if the goal was to produce a file with
an empty RPATH.  Restore this tolerance in order to support setting an
empty RPATH on GNU ld scripts.

Fixes: #22963