[core] signing.passphrase must not be blank.

In a `signing {}` block like so:

```
    jreleaser {
        signing {
            setActive("ALWAYS")
            setMode("COMMAND")
        }
        ...
    }
```

I get the error:

```
[ERROR] signing.passphrase must not be blank.
```

But what if the PGP private key is not protected by a passphrase? It feels like jreleaser should not be enforcing it, should it?