Fix push for subsequent subscriptions

[tarikeshaq]

fixes #4691

Uses the client cached uaid when the server doesn't return one. If we didn't have a uaid, we error out, because we cannot return the subscription back to our consumer.

From the logs, it seems like we are hitting this only on subsequent subscriptions (i.e not the first one where we got the uaid)

I'll need to cut a release once this merges

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[codecov-commenter]

Codecov Report

Merging #4697 (21cc0eb) into main (5981760) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4697   +/-   ##
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  Coverage   80.53%   80.53%           
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  Files          48       48           
  Lines        5220     5220           
=======================================
  Hits         4204     4204           
  Misses       1016     1016           

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[tarikeshaq]

Tested this locally and verified that it works (and that it was broken!!!)
How to repro if anyone is interested:

• On fenix, sign in to Firefox accounts, this will create the first subscription
• then, navigate to https://webpushdemo.azurewebsites.net/ (easy way to test web-push, the push notification will come in 5 seconds) and click on "initiate push"
• quickly minimize the app,
• 5 seconds later, nothing happens if you're on the latest nightly

With this patch on top of 82.2.0 (note that the latest main has breaking changes for fenix, so I'll create a dot-release on 82.2 that includes only this patch)
it works and 5 seconds later the push notification shows up, even if the app is minimized