fix: throw when adc cannot acquire a projectId

[JustinBeckwith]

BREAKING CHANGE Currently, when a user is using application default credentials, and they call auth.getProjectId, we check a variety of sources, and return the first projectId we find. In the event of none - we return null. This PR changes the behavior, so instead of returning null, we throw an exception.

[codecov]

Codecov Report

Merging #658 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
+ Coverage   88.12%   88.25%   +0.12%     
==========================================
  Files          18       18              
  Lines         783      783              
  Branches       84       84              
==========================================
+ Hits          690      691       +1     
+ Misses         82       81       -1     
  Partials       11       11

Impacted Files	Coverage Δ
src/auth/googleauth.ts	91.28% <100%> (+0.51%)	⬆️

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[codecov]

Codecov Report

Merging #658 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
+ Coverage   88.12%   88.25%   +0.12%     
==========================================
  Files          18       18              
  Lines         783      783              
  Branches       84       84              
==========================================
+ Hits          690      691       +1     
+ Misses         82       81       -1     
  Partials       11       11

Impacted Files	Coverage Δ
src/auth/googleauth.ts	91.28% <100%> (+0.51%)	⬆️

---

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 48a1c81...b0fa725. Read the comment docs.

[bcoe]

I like this a lot 👍

[bcoe]

I like this addition, I'd way rather the library fail early than run in an undefined state with no project ID.

[JustinBeckwith]