Do not try to update EFI variables if empty or read only

[travier]

According to https://bugzilla.redhat.com/show_bug.cgi?id=2333190, systems sometimes have read only EFI variables and thus any efibootmgr command would fail to update them. We should either:

• make this command fail open (i.e. always ignore efibootmgr failures)
• or check for the read only state and skip it

See also the following issues that are likely related:

• Log more information when invoking efibootmgr #907 & Add more log when invoking efibootmgr #951
• Wont boot on Pinebook Pro fedora-silverblue/issue-tracker#645
• Failed to write boot loader configuration with Silverblue 41 on Framework 13 fedora-silverblue/issue-tracker#614