sleep: do not abort if we try to query capacity of missing battery

sleep: do not abort if we try to query capacity of missing battery

Fixes #25584.

From the issue:
Assertion 'capacity >= 0' failed at src/shared/sleep-config.c:58, function PTR_TO_CAPACITY(). Aborting.

(gdb) bt

The problem is that PTR_TO_CAPACITY(hashmap_get(last_capacity, battery_name))
will abort if it's called with a name not present in the hashmap. We want to
skip the device silently in this case instead.