Fix NPE in SignatureVerifier when JWKS key is not found

Author: utkrishtsahu

auth0/src/main/java/com/auth0/android/provider/SignatureVerifier.java

@@ -57,6 +57,10 @@ static void forAsymmetricAlgorithm(@Nullable final String keyId, @NonNull Authen
             @Override
             public void onSuccess(@Nullable Map<String, PublicKey> result) {
                 PublicKey publicKey = result.get(keyId);
+                if (publicKey == null) {
+                    callback.onFailure(new PublicKeyNotFoundException(keyId));
+                    return;
+                }
                 try {
                     callback.onSuccess(new AsymmetricSignatureVerifier(publicKey));
                 } catch (InvalidKeyException e) {