fix: landing when no Jenkins CI has been run for PR

[codebytere]

Closes #452.

Fixes the following error:

------------------------------ Generated metadata ------------------------------
PR-URL: 
Backport-PR-URL: https://github.com/nodejs/node/pull/33126
Refs: https://github.com/nodejs/node/pull/32763
--------------------------------------------------------------------------------
   ℹ  Last Full PR CI on 2020-04-28T15:56:53Z: https://ci.nodejs.org/job/node-test-pull-request/31061/
   ⚠  Commits were pushed after the last Full PR CI run:
   ⚠  - stream: runtime deprecate Transform._transformState
✔  Build data downloaded
{
  servlet: 'Stapler',
  message: 'Not Found',
  url: '/job/node-test-pull-request/31061/api/json',
  status: '404'
}
TypeError: Cannot read property 'find' of undefined
    at PRBuild.setBuildData (/Users/codebytere/Developer/node-core-utils/lib/ci/ci_result_parser.js:208:28)
    at PRBuild.getResults (/Users/codebytere/Developer/node-core-utils/lib/ci/ci_result_parser.js:696:10)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async PRChecker.checkJenkinsCI (/Users/codebytere/Developer/node-core-utils/lib/pr_checker.js:290:36)
    at async PRChecker.checkCI (/Users/codebytere/Developer/node-core-utils/lib/pr_checker.js:219:16)
    at async PRChecker.checkAll (/Users/codebytere/Developer/node-core-utils/lib/pr_checker.js:75:7)
    at async getMetadata (/Users/codebytere/Developer/node-core-utils/components/metadata.js:43:18)
    at async main (/Users/codebytere/Developer/node-core-utils/components/git/land.js:163:22)

caused when no Jenkins CI has been run in so long that it's a 404. Seen when trying to land https://ci.nodejs.org/job/node-test-pull-request/31061/.

cc @targos @mmarchini

[codecov]

Codecov Report

Merging #451 into master will decrease coverage by 0.13%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   77.08%   76.94%   -0.14%     
==========================================
  Files          21       21              
  Lines        1497     1501       +4     
==========================================
+ Hits         1154     1155       +1     
- Misses        343      346       +3     

Impacted Files	Coverage Δ
lib/ci/ci_result_parser.js	47.69% <40.00%> (-0.19%)	⬇️

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