fix(ui): stops cached image from being used when channel image is not set

[deven98]

Fixes #1931

ChannelHeader no longer uses CachedNetworkImage if channel image is not set.

[Brazol]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.26%. Comparing base (ff924da) to head (5521f35).

Files	Patch %	Lines
...flutter/lib/src/channel/stream_channel_avatar.dart	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1949      +/-   ##
==========================================
+ Coverage   60.23%   60.26%   +0.02%     
==========================================
  Files         317      317              
  Lines       18493    18495       +2     
==========================================
+ Hits        11140    11146       +6     
+ Misses       7353     7349       -4     

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