[RFR] 8243424: Signature and SignatureSpi get parameter methods may throw null when unsupported

[Anthony Scarpino]

Hi,

I need a javadoc review for a Signature/SignatureSpi parameters change 
that is needed to the spec, which affects EdDSA.  The existing spec was 
very strict in the way set and get parameters were handled.  This 
modifies that slightly by allowing null to be returned by the get 
methods when it is unsupported after the parameters have been set. 
Since null was an allowed return value, this minimizes the backwards 
compatibility chances.

CSR: https://bugs.openjdk.java.net/browse/JDK-8243425
webrev: https://cr.openjdk.java.net/~ascarpino/8243424/webrev/

Tony