opencensus-zpages: Enforce strictNullChecks and noUnusedLocals

[mayurkale22]

This is part of #348. 3 more packages to go, 19 done so far.

[draffensperger]

Nice!

[codecov-io]

Codecov Report

Merging #432 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #432      +/-   ##
=========================================
- Coverage   94.94%   94.9%   -0.04%     
=========================================
  Files         141     141              
  Lines        9334    9321      -13     
  Branches      672     673       +1     
=========================================
- Hits         8862    8846      -16     
- Misses        472     475       +3

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	80.2% <0%> (-3.13%)	⬇️
...rontend/page-handlers/traceconfigz.page-handler.ts	70.83% <0%> (ø)	⬆️
...zpages-frontend/page-handlers/rpcz.page-handler.ts	100% <0%> (ø)	⬆️
test/test-zpages.ts	99.62% <0%> (ø)	⬆️
src/zpages.ts	87.93% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d838da0...b2efd48. Read the comment docs.