Handle CMsgClientMMSLobbyData with null metadata

[Benjamin-Dobell]

Closes #826

[azuisleet]

The assertion cachedLobby != null in HandleLobbyData, is that still correct for a lobby we discover through friend presence data? Will that normally be hit for such a case?

[codecov]

Codecov Report

Merging #828 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #828   +/-   ##
=======================================
  Coverage   22.77%   22.77%           
=======================================
  Files          95       95           
  Lines        9331     9330    -1     
  Branches      772      779    +7     
=======================================
  Hits         2125     2125           
+ Misses       7072     7071    -1     
  Partials      134      134           

Impacted Files	Coverage Δ
...SteamKit2/Steam/Handlers/SteamMatchmaking/Lobby.cs	0.00% <0.00%> (ø)
...team/Handlers/SteamMatchmaking/SteamMatchmaking.cs	3.77% <0.00%> (+<0.01%)	⬆️

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[Benjamin-Dobell]

Yeah, I don't believe it was ever accurate actually, it was added in:

c67f2ab#diff-afcd8513c5d05604d1e1610042a9f576R591

I'll add in a commit to this PR removing it.

[azuisleet]

Ah yes, the assertion was for the previously non-nullable access to Members.