Fix generic claims not signing (#46)

When calling DoEncode for GenericClaims the expected prefixes length of
0 is stopping the signing and throwing an exception. We can skip the
check when expected prefixes length=0.

Author: darkwatchuk

src/NATS.Jwt/NatsJwt.cs

@@ -475,7 +475,7 @@ private static string DoEncode<T>(JwtHeader jwtHeader, KeyPair keyPair, T claim,
 
         string issuer = keyPair.GetPublicKey();
         PrefixByte[] expectedPrefixes = c.ExpectedPrefixes();
-        if (!expectedPrefixes.Any(p => KeyPair.IsValidPublicKey(p, issuer.AsSpan())))
+        if (expectedPrefixes.Length > 0 && !expectedPrefixes.Any(p => KeyPair.IsValidPublicKey(p, issuer.AsSpan())))
         {
             throw new NatsJwtException($"Invalid signing key of '{keyPair.Prefix}': expected one of '{string.Join(",", expectedPrefixes)}'");
         }