Fix null check for public key parameter.

[dkocher]

Addendum for #908 as discussed in ¹. Resolves #907.

Footnotes

1. https://github.com/hierynomus/sshj/pull/908/files#r1600044608 ↩

[hierynomus]

Actually the null check is not for the parameter, but was once meant to only ensure that a subsequent call to init would not re-initialize. Need to check what the potential problem is.