mappingConfig fails to pull

[indig0fox]

This call was intermittently failing for me and causing the playback page to get stuck on "Loading" the replay https://github.com/alexcroox/R3-Web/blob/ac92ae0ac3655c051eef5d5685d843bdfc829b95/dist/playback.php#L27-L32

So I had to add a null check in playback.php

if (!$mappingConfig)
	$mappingConfig = '{}';