repart: allow empty EncryptedVolume= volume name (#42889)

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.

diff --cc src/repart/repart.c
Simple merge

diff --cc test/units/TEST-58-REPART.sh
Simple merge