bug(http): npe when getting non initialized workers

[rustatian]

Reason for This PR

closes: #143

Description of Changes

• Check if the HTTP pool is ready before trying to return workers from it.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.] [Reviewer TODO: Verify that these criteria are met. Request changes if not]

• [x] All commits in this PR are signed (git commit -s).
• [x] The reason for this PR is clearly provided (issue no. or explanation).
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [x] Any user-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.

[codecov[bot]]

Codecov Report

Merging #145 (209a01d) into master (85760eb) will increase coverage by 0.44%. The diff coverage is 57.14%.

:exclamation: Current head 209a01d differs from pull request most recent head f4a5fce. Consider uploading reports for the commit f4a5fce to get more accurate results

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   64.93%   65.38%   +0.44%     
==========================================
  Files         139      139              
  Lines       11043    11045       +2     
==========================================
+ Hits         7171     7222      +51     
+ Misses       3143     3091      -52     
- Partials      729      732       +3     

Impacted Files	Coverage Δ
http/metrics.go	100.00% <ø> (ø)
http/plugin.go	75.52% <0.00%> (-1.70%)	:arrow_down:
jobs/metrics.go	100.00% <ø> (ø)
reload/plugin.go	72.54% <ø> (+0.70%)	:arrow_up:
resetter/plugin.go	85.71% <ø> (+3.89%)	:arrow_up:
rpc/plugin.go	76.19% <ø> (+0.89%)	:arrow_up:
informer/plugin.go	71.05% <100.00%> (ø)
informer/rpc.go	64.00% <100.00%> (ø)
tcp/handler/handler.go	51.35% <0.00%> (-6.09%)	:arrow_down:
... and 7 more

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