Add null check for possible NPE

Author: janakamarasena

components/org.wso2.carbon.identity.oauth.endpoint/src/main/java/org/wso2/carbon/identity/oauth/endpoint/api/auth/ApiAuthnHandler.java

@@ -131,7 +131,9 @@ private AuthenticatorMetadata buildAuthenticatorMetadata(AuthenticatorData authe
                 params.add(param);
             }
             authenticatorMetadata.setParams(params);
-            authenticatorMetadata.setAdditionalData(getAdditionalData(authenticatorData.getAdditionalData()));
+            if (authenticatorData.getAdditionalData() != null) {
+                authenticatorMetadata.setAdditionalData(getAdditionalData(authenticatorData.getAdditionalData()));
+            }
         }
         return authenticatorMetadata;
     }