smugmug_fs: gracefully degrade for video files with no mtime

[durin42]

Sometimes (as with the renames, I have no idea what triggers this) SmugMug doesn't record a DateTimeModified for a video (presumably other filetypes too, but we use MD5 checks for most other files) and we would crash when checking the empty-string that we got back. Instead just assume that the file isn't modified and move on.