refactor: Allow NULL pointers for byte arrays in events.

[iphydf]

Some events, notably the file chunk recv one, can give NULL pointers to the client. Not sure they should, but that's what happens, so we support it.

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This change is 

[codecov]

Codecov Report

Attention: 63 lines in your changes are missing coverage. Please review.

Comparison is base (5e2c8ca) 66.90% compared to head (b96e1bd) 66.78%.

❗ Current head b96e1bd differs from pull request most recent head e7fb91d. Consider uploading reports for the commit e7fb91d to get more accurate results

Files	Patch %	Lines
toxcore/events/group_invite.c	0.00%	6 Missing ⚠️
toxcore/events/group_peer_exit.c	0.00%	6 Missing ⚠️
toxcore/events/conference_invite.c	0.00%	3 Missing ⚠️
toxcore/events/conference_message.c	0.00%	3 Missing ⚠️
toxcore/events/conference_peer_name.c	0.00%	3 Missing ⚠️
toxcore/events/conference_title.c	0.00%	3 Missing ⚠️
toxcore/events/file_recv.c	0.00%	3 Missing ⚠️
toxcore/events/file_recv_chunk.c	0.00%	3 Missing ⚠️
toxcore/events/friend_lossless_packet.c	0.00%	3 Missing ⚠️
toxcore/events/friend_lossy_packet.c	0.00%	3 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2578      +/-   ##
==========================================
- Coverage   66.90%   66.78%   -0.13%     
==========================================
  Files         148      148              
  Lines       30262    30328      +66     
==========================================
+ Hits        20247    20254       +7     
- Misses      10015    10074      +59     

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