Apply "fix: do not crash on pool with no volumes"

[rustybird]

Submitted by @ethnh
Authored by @CertainLach
Fixes QubesOS/qubes-issues#9504

I don't know what exactly was crashing there. But the patch LGTM, and I've checked that list(qubesadmin.Qubes().pools['my-empty-pool'].volumes) now correctly results in [] instead of raising AssertionError.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.09%. Comparing base (30d7547) to head (54b7428).

Files with missing lines	Patch %	Lines
qubesadmin/storage.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   75.10%   75.09%   -0.01%     
==========================================
  Files          52       52              
  Lines        8552     8554       +2     
==========================================
+ Hits         6423     6424       +1     
- Misses       2129     2130       +1     

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[CertainLach]

Qubesd and qubes-qube-manager are not handling this AssertionError, resulting in crash.

I think this error might be specific to nixos-qubes bootstrapping, where the initial system doesn't even have kernels available, but still this AssertionError is not good

[rustybird]

Makes sense that the crash was in qubesmanager: That was the only consumer of the buggy method I found too, and limited to kernel pools. (qvm-volume list --pool=... calls a different method)