internal/safefilepath: fix TestFromFS on Plan 9
    
    CL 455716 added TestFromFS. This test was failing on Plan 9
    because fromFS didn't return an empty string in case of error.
    
    This change fixes TestFromFS by returning an empty string
    in case of error.
    
    Fixes #57142.
    
    Change-Id: Ie50dfba5e70154d641f762fa43f1c26c3d12b6f6
    Reviewed-on: https://go-review.googlesource.com/c/go/+/455835
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@google.com>
    Reviewed-by: Damien Neil <dneil@google.com>
    Run-TryBot: David du Colombier <0intro@gmail.com>