Fix check for whether metadata encryption is enabled

metadata_key_dir controls whether metadata encryption is enabled, not
metadata_encryption which is just the encryption options and can be
empty if the default options are sufficient.

Bug: 198405417
Fixes: 156d9d22 ("Pre-create userdata metadata encryption device.")
Change-Id: I98ee253c7e5e738ee087ec07ed60f4a376ca7cee