SCB-472 Null point reference in zipkin plugin

[little-cui]

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[coveralls]

Coverage decreased (-0.3%) to 73.039% when pulling 9400811 on little-cui:master into 7650f35 on apache:master.

[codecov-io]

Codecov Report

Merging #325 into master will decrease coverage by 0.24%.
The diff coverage is 75.67%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   69.73%   69.48%   -0.25%     
==========================================
  Files          17       17              
  Lines        3426     3438      +12     
==========================================
  Hits         2389     2389              
- Misses        871      884      +13     
+ Partials      166      165       -1

Impacted Files	Coverage Δ
server/service/util/dependency.go	56.54% <100%> (-1.24%)	⬇️
server/service/microservices.go	74.55% <63.15%> (-0.05%)	⬇️
server/service/instances.go	68.32% <85.71%> (-0.07%)	⬇️
server/service/tag.go	76.02% <0%> (+0.58%)	⬆️

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