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
32 jobs for !6779 with rpath-unrecognized-format in 30 minutes and 3 seconds (queued for 2 seconds)
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