e.g. when we first create it
because it throws this error:
```
panic: assignment to entry in nil map
goroutine 1 [running]:
github.com/kairos-io/kcrypt/pkg/partition_info.PartitionInfo.UpdateMapping({{0xa60325?, 0x18?}, 0x0?}, {0xc000302000?, 0x1?})
/go/pkg/mod/github.com/kairos-io/kcrypt@v0.4.2/pkg/partition_info/partition_info.go:69 +0x99
```
Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>
so that the caller doesn't have to care if it's the first time the
partition is encrypted or not.
Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>
Now the code can simply initialize a PartitionInfo from a file and then
call LookupUUIDForLabel on it.
Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>
to be used both here (when trying to find the partition UUID using a
label) and on the kairos side when updating the file after calling
kcrypt to encrypt a partition (which causes the UUID to change).
Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>
