fix(channel): reset state to EmptyChannelState after unsetting

[SOF3]

fixes #200

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09 🎉

Comparison is base (9e07d5a) 93.04% compared to head (8c61da5) 93.14%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #201      +/-   ##
============================================
+ Coverage     93.04%   93.14%   +0.09%     
- Complexity      157      161       +4     
============================================
  Files            12       12              
  Lines           446      452       +6     
============================================
+ Hits            415      421       +6     
  Misses           31       31              

Impacted Files	Coverage Δ
...wait-generator/src/SOFe/AwaitGenerator/Channel.php	100.00% <100.00%> (ø)

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