Filter unavailable users and remove unavailable shares on polling

[VicDeo]

Description

Protect polling from exploding on unavailable users

Motivation and Context

An unhandled exception has been thrown: TypeError: Argument 1 passed to OCA\Files_Sharing\External\MountProvider::getMountsForUser() must implement interface OCP\IUser, null given, when UserManager knows nothing about a userId

How Has This Been Tested?

1. Create and accept a federated share
2. Execute update oc_share_external set user="Iwouldliketobreakthepolling" limit 1;

Expected

Polling goes through all entries with no termination

Actual

Polling terminates with TypeError: Argument 1 passed to OCA\Files_Sharing\External\MountProvider::getMountsForUser() must implement interface OCP\IUser, null given

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

Open tasks:

• Backport (if applicable set "backport-request" label and remove when the backport was done)

[codecov]

Codecov Report

Merging #35010 into master will increase coverage by <.01%.
The diff coverage is 79.31%.

@@             Coverage Diff              @@
##             master   #35010      +/-   ##
============================================
+ Coverage     65.37%   65.37%   +<.01%     
- Complexity    18622    18626       +4     
============================================
  Files          1215     1215              
  Lines         70510    70532      +22     
  Branches       1295     1295              
============================================
+ Hits          46095    46111      +16     
- Misses        24041    24047       +6     
  Partials        374      374

Flag	Coverage Δ	Complexity Δ
#javascript	52.85% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	66.82% <79.31%> (ø)	18626 <3> (+4)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
...s/federatedfilesharing/lib/AppInfo/Application.php	45.89% <0%> (-0.97%)	19 <0> (ø)
...atedfilesharing/lib/Command/PollIncomingShares.php	98.52% <100%> (+0.41%)	13 <2> (+2)	⬆️
apps/files_sharing/lib/External/Manager.php	76.54% <40%> (-1.31%)	32 <1> (+2)
apps/files_sharing/js/PublicUploadView.js	67.39% <0%> (ø)	0% <0%> (ø)	⬇️

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[PVince81]

👍

[PVince81]

@VicDeo please backport to release-10.2.0 branch

[VicDeo]

release-10.2.0: #35073

[phil-davis]

This was backported into release-10.2.0 which was then merged into stable10 - so it is part of stable10 release series.