Resources: Fix getting for an unknown id

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Fixes an error that occurs if get is used for an id that doesn't exist in the resource cache.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit d2dd6d3:

Sandbox	Source
dojo/dojo-codesandbox-template	Configuration

[codecov]

Codecov Report

Merging #884 (80a3392) into master (c3b4866) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #884   +/-   ##
=======================================
  Coverage   94.52%   94.52%           
=======================================
  Files         127      127           
  Lines        8094     8095    +1     
  Branches     1888     1889    +1     
=======================================
+ Hits         7651     7652    +1     
  Misses        443      443           

Impacted Files	Coverage Δ
src/core/middleware/resources.ts	95.69% <100.00%> (+0.01%)	⬆️

---

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 c3b4866...d2dd6d3. Read the comment docs.