fix: Fix NPE when a conference request has no payload

[bgrozev]

Jetty doesn't recognize that the kotlin param can't be null which
results in a NPE and response 500. Return 400 without polluting the logs
instead.

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (660d705) 27.64% compared to head (0682a6c) 28.93%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1126      +/-   ##
============================================
+ Coverage     27.64%   28.93%   +1.29%     
- Complexity      468      475       +7     
============================================
  Files           128      128              
  Lines          7800     7818      +18     
  Branches       1066     1069       +3     
============================================
+ Hits           2156     2262     +106     
+ Misses         5373     5284      -89     
- Partials        271      272       +1     

Files	Coverage Δ
.../main/kotlin/org/jitsi/jicofo/xmpp/muc/ChatRoom.kt	0.00% <ø> (ø)
...ofo/src/main/kotlin/org/jitsi/jicofo/xmpp/Smack.kt	100.00% <100.00%> (+100.00%)	⬆️
.../kotlin/org/jitsi/jicofo/rest/ConferenceRequest.kt	0.00% <0.00%> (ø)
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	44.40% <0.00%> (+1.09%)	⬆️
...n/kotlin/org/jitsi/jicofo/xmpp/muc/ChatRoomImpl.kt	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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