fix: do not hang in LRO forever if API returned nothing

[alexander-fenster]

Speech API LongRunningRecognize can return no result and no error in case if the input file has no speech in it. This should be fixed on the server side but it makes sense to stop waiting in LRO if there is nothing to wait for.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #436 into master will decrease coverage by 0.13%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
- Coverage    92.8%   92.67%   -0.14%     
==========================================
  Files          26       26              
  Lines        2753     2757       +4     
  Branches       72       72              
==========================================
  Hits         2555     2555              
- Misses        186      190       +4     
  Partials       12       12

Impacted Files	Coverage Δ
src/longrunning.ts	94.64% <0%> (-3.51%)	⬇️

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[codecov]

Codecov Report

Merging #436 into master will increase coverage by <.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage    92.8%   92.81%   +<.01%     
==========================================
  Files          26       26              
  Lines        2753     2769      +16     
  Branches       72       72              
==========================================
+ Hits         2555     2570      +15     
- Misses        186      187       +1     
  Partials       12       12

Impacted Files	Coverage Δ
src/longrunning.ts	98.21% <100%> (+0.06%)	⬆️
test/longrunning.ts	93.61% <91.66%> (-0.08%)	⬇️

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