user is undefined when name is not a current user

[frejos]

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• [N/A] I've written tests to cover the new code and functionality included in this PR.
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PR Summary

Calling SlackMemoryDataStore.prototype.getDMByName with a name that is not a current user should return undefined, but currently throws a TypeError.

return find(this.dms, ['user', user.id]);
^
TypeError: Cannot read property 'id' of undefined

Simple fix to check for existence or user prior to doing search and appropriately returning undefined if it does not.

Related Issues

Did not open issue.

Test strategy

No additional or changed functionality, searching for name that is not a current user.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #332 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   85.72%   85.72%           
=======================================
  Files          44       44           
  Lines        1198     1198           
  Branches      175      176    +1     
=======================================
  Hits         1027     1027           
  Misses        171      171

Impacted Files	Coverage Δ
lib/data-store/memory-data-store.js	80.35% <100%> (ø)	⬆️

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[aoberoi]

good catch, thanks @frejos!