MetadataParser: Don't fail hard when a thumbnail isn't an actual path

(or more accurately when its mrl doesn't represent a local path)
4 jobs for various_parser_fix in 11 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #286406
amd64 debian
asan-ubsan

00:11:57

passed #286403
amd64 debian
linux

00:08:51

passed #286404
win32
win32

00:10:46

passed #286405
win64
win64

00:11:13