Treat an empty volume name alongside other fields as unset instead of
rejecting it as invalid.
Example use case:
```
EncryptedVolume=:none:discard
```
In this case, the volume name is not specified so it can be generated as
luks-UUID.
From the docs:
> EncryptedVolume=
> Specifies how the encrypted partition should be set up. Takes at least
one and at most four fields separated with a colon (":"). The first
field specifies the encrypted volume name under /dev/mapper/. If not
specified, "luks-UUID" will be used where "UUID" is the LUKS UUID.
Lennart Poettering
6ce0759d99