Do not die if missing revision metadata.

Do not die if missing revision metadata.

The Xonotic revision metadata comes as a part of the 'R'
line in the POST. Although it isn't required, server was dying
when it was missing. I made it optional now, defaulting
that field to "unkown" when it isn't present. Note: DTG
brought this issue into the light, as they custom compile
their Xonotic, and they omit #ifdef'ing WARPZONE, which
is what prints the 'R' line in stats.