Handle empty Z-Stack build ID in the version response

[puddly]

Fixes #116

[codecov-commenter]

Codecov Report

Merging #117 (8cdcb74) into dev (074f33f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #117   +/-   ##
=======================================
  Coverage   98.73%   98.73%           
=======================================
  Files          44       44           
  Lines        3873     3876    +3     
=======================================
+ Hits         3824     3827    +3     
  Misses         49       49           

Impacted Files	Coverage Δ
zigpy_znp/zigbee/application.py	96.22% <100.00%> (+0.02%)	⬆️

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