Fix trailing space in rtcp-fb with no Parameter

[Sean-Der]

Media construction would always do "%s %s" even if no Parameter was available. This commit fixes that by checking if Parameter is empty or not.

Resolves #3207

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.31%. Comparing base (457679c) to head (7809b8a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3218      +/-   ##
==========================================
+ Coverage   78.27%   78.31%   +0.03%     
==========================================
  Files          93       93              
  Lines       11906    11909       +3     
==========================================
+ Hits         9320     9327       +7     
+ Misses       2073     2071       -2     
+ Partials      513      511       -2     

Flag	Coverage Δ
go	80.09% <100.00%> (+0.04%)	⬆️
wasm	63.22% <ø> (ø)

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