pubKeyHash.toJson leads to NullPointerException

[sitaram-kalluri]

Describe the bug

In at_client, the monitor connection fails to start with the below exception

WARNING|2024-02-21 10:29:34.818178|Monitor (@alice🛠)|runZonedGuarded received socket error Converting object to an encodable object failed: Instance of 'Metadata' - calling _handleError 

INFO|2024-02-21 10:29:34.818691|Monitor (@alice🛠)|socket.listen onDone called. Will destroy socket, set status stopped, call retryCallback 

INFO|2024-02-21 10:29:34.818936|Monitor (@alice🛠)|Monitor error Converting object to an encodable object failed: Instance of 'Metadata' - calling the retryCallback 

The reason for the above failure is because the Metadata object fails JSON Encode due to the NullPointerException. Attaching the error stack trace

#0      Metadata.toJson (package:at_commons/src/keystore/at_key.dart:687:60)
#1      _defaultToEncodable (dart:convert/json.dart:627:55)
#2      _JsonStringifier.writeObject (dart:convert/json.dart:787:36)
#3      _JsonStringifier.writeMap (dart:convert/json.dart:874:7)
#4      _JsonStringifier.writeJsonValue (dart:convert/json.dart:829:21)
#5      _JsonStringifier.writeObject (dart:convert/json.dart:784:9)
#6      _JsonStringStringifier.printOn (dart:convert/json.dart:982:17)
#7      _JsonStringStringifier.stringify (dart:convert/json.dart:967:5)
#8      JsonEncoder.convert (dart:convert/json.dart:345:30)
#9      JsonCodec.encode (dart:convert/json.dart:231:45)
#10     jsonEncode (dart:convert/json.dart:114:10)
#11     NotificationServiceImpl._internalNotificationCallback (package:at_client/src/service/notification_service_impl.dart:236:15)
<asynchronous suspension>

Below condition in at_key.dart is the root cause the above mentioned exceptions. The default value of fullJson is true. Since fullJson is true and the condtion is "OR", even if pubKeyHash is "NULL", the control still enters into IF condition and toJson() is invoked on NULL object leading to the above exceptions.

if (fullJson || pubKeyHash != null) {
      map[AtConstants.sharedWithPublicKeyHash] = pubKeyHash!.toJson();
    }

Steps to reproduce

1. Point at_commons version to 4.0.2
2. Run the tests related to monitor connection.
3. For instance: test/enrollment_test.dart: A test to verify enrollment request returns notification in at_client functional test.

Expected behavior

The Monitor connection should be started successfully.