Fix marshalling of raw_message field for empty messages

[bradleyjkemp]

Always marshal the raw_message field. When marshaling (i.e. from grpc-dump), this field should always be written even if it is the empty string.

Fixes #105

[codecov]

Codecov Report

Merging #106 into master will decrease coverage by 0.09%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   56.49%   56.40%   -0.10%     
==========================================
  Files          31       31              
  Lines        1177     1179       +2     
==========================================
  Hits          665      665              
- Misses        408      409       +1     
- Partials      104      105       +1     

Flag	Coverage Δ
#integration	55.81% <0.00%> (-0.10%)	⬇️
#unit	18.67% <ø> (ø)

Impacted Files	Coverage Δ
grpc-dump/dump/recorded_server_stream.go	82.50% <0.00%> (-4.35%)	⬇️
internal/dump.go	100.00% <ø> (ø)

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