Leaving output (--out) param empty raises exception when generating icons

[sovietspaceship]

--out is meant to be optional, but omitting it raises the following exception:

Sourcing icons failed: Invalid destination scheme

From https://github.com/EldenRingDatabase/erdb/blob/831d5e6af04a99a9aceedccdfb4c796db42e4dc0/src/erdb/utils/sourcer.py#L401