8204152: SignedObject throws NullPointerException for null keys with an initialized Signature object

Check for null and throw InvalidKeyException to maintain same behavior

Reviewed-by: xuelei

src/java.base/share/classes/sun/security/rsa/RSAKeyFactory.java

@@ -114,6 +114,9 @@ public class RSAKeyFactory extends KeyFactorySpi {
      * Used by RSASignature and RSACipher.
      */
     public static RSAKey toRSAKey(Key key) throws InvalidKeyException {
+        if (key == null) {
+            throw new InvalidKeyException("Key must not be null");
+        }
         if ((key instanceof RSAPrivateKeyImpl) ||
             (key instanceof RSAPrivateCrtKeyImpl) ||
             (key instanceof RSAPublicKeyImpl)) {