set default of SendResult.Status and fix panic

[dbl-x]

1. set the default value of SendResult.Status to SendUnknownError
2. fix panic: select queue with nil

[codecov-commenter]

Codecov Report

Merging #495 into master will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
- Coverage   24.80%   24.76%   -0.04%     
==========================================
  Files          49       49              
  Lines        5716     5725       +9     
==========================================
  Hits         1418     1418              
- Misses       4095     4104       +9     
  Partials      203      203              

Impacted Files	Coverage Δ
internal/trace.go	27.05% <0.00%> (-0.67%)	⬇️
primitive/result.go	0.00% <0.00%> (ø)
producer/producer.go	29.30% <66.66%> (ø)

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[xujianhai666]

lgtm