Reject negative JRuby key lengths consistently

[OskarEichler]

Summary:

• reject every key length below one in the JRuby implementation
• extend the shared invalid-argument coverage to negative lengths

Why:
The Java implementation checks only keylen == 0. A negative key length therefore reaches new byte[keylen] and raises NegativeArraySizeException, while the MRI boundary rejects out-of-range lengths. Treating keylen < 1 as invalid keeps the Java implementation inside the method contract and returns nil consistently.

Verification:

• Java model reproduces NegativeArraySizeException before the change and nil afterward
• shared current suite remains green on MRI: 8 runs, 31 assertions
• existing jBCrypt vectors, including the 64-byte multi-block case, pass

Compatibility:
Positive key lengths are unchanged. Negative low-level inputs now return nil instead of leaking a Java allocation exception.