UI: Check low disk space only if recording to a file

The Custom FFmpeg Output allows the user to configure it to output to a
URL instead of a file. In OBSBasic::StartRecording(), we unconditionally
call LowDiskSpace() to check for low disk space because we assume that a
"recording" will always be to disk. When os_get_free_disk_space() is
called on a non-existent path, it returns 0, which causes OBS to emit a
low disk space warning. Instead, only call DiskSpaceMessage() if not
"recording" to a URL.
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