add default value when attributes are not specified

[mayurkale22]

This will fix the issue when we export data to Stackdriver backend https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-exporter-stackdriver/src/stackdriver-cloudtrace-utils.ts#L120, otherwise the program will crash when optional attributes is missing.

I have open an issue #348 to enable strictNullChecks on this repo.

[codecov-io]

Codecov Report

Merging #351 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   95.23%   95.27%   +0.03%     
==========================================
  Files         122      122              
  Lines        8277     8288      +11     
  Branches      608      610       +2     
==========================================
+ Hits         7883     7896      +13     
+ Misses        394      392       -2

Impacted Files	Coverage Δ
src/adapters.ts	96% <0%> (-1%)	⬇️
test/test-root-span.ts	100% <0%> (ø)	⬆️
test/test-ocagent.ts	91.74% <0%> (ø)	⬆️
src/trace/model/span-base.ts	98.79% <0%> (+0.02%)	⬆️
src/stackdriver-monitoring.ts	84.21% <0%> (+3.15%)	⬆️

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