service: Workaround for error on s390x (#1822)

## Problem

As described at #1818, there seems to be a problem when installing at
s390x and starting with no available disks. See [this
comment](#1818 (comment))
at the original issue.

After visiting the DASD section and activating+formatting a device, the
backend fails and the UI becomes barely usable.

The error is produced when checking `storage_manager.proposal` after
re-probing the system and trying an (unsuccessful) proposal.

## Solution

This pull request does not implement a real fix, but a workaround that
has proven to be useful in that case. It allowed the installation to
continue successfully at the reported scenario.

I tried to fix the real root of the issue, but I was unable to find out
why `storage_manager.proposal` was nil at that point.

But since we are rewriting the approach to the storage proposal, the
whole `ProposalStrategies::Guided` class is going to disappear so it
makes little sense to invest more time.

I will keep the original issue open and will create a Trello card to
track it, so we check whether the new implementation of the storage
proposal (still under development) is reliable in that situation.

## Testing

Verified with manual testing. Going further would be wasting resources
right now.

service/lib/agama/storage/proposal_strategies/guided.rb

@@ -62,6 +62,9 @@ def calculate
 
         # @see Base#issues
         def issues
+          # Returning [] in case of a missing proposal is a workaround (the scenario should
+          # not happen). But this class is not expected to live long.
+          return [] unless storage_manager.proposal
           return [] unless storage_manager.proposal.failed?
 
           [target_device_issue, missing_devices_issue].compact

service/package/rubygem-agama-yast.changes

@@ -1,3 +1,9 @@
+-------------------------------------------------------------------
+Mon Dec  9 14:43:11 UTC 2024 - Ancor Gonzalez Sosa <[email protected]>
+
+- Work around to a problem triggered after formatting DASD devices
+  (mitigates gh#agama-project/agama#1818).
+
 -------------------------------------------------------------------
 Tue Dec  3 20:32:08 UTC 2024 - Josef Reidinger <[email protected]>