NPE in MultiPrimaryReplicaClient

It seems there is NPE path in code of `MultiPrimaryReplicaClient`.
Precondition - when there was an issue with replica and

1. `setReadOnly(true)` is called the inner state will be requestReadOnly=true, replicaClient=null, primaryClient != null
2. when `setReadOnly(false)` is called later it will fail with NPE as replicaClient is null

Line https://github.com/mariadb-corporation/mariadb-connector-j/blob/5222bfb7b30f8bcb71447b03d86843e872d19ec2/src/main/java/org/mariadb/jdbc/client/impl/MultiPrimaryReplicaClient.java#L294

```
java.lang.NullPointerException: Cannot invoke "org.mariadb.jdbc.client.Client.getContext()" because "oldCli" is null
at org.mariadb.jdbc.client.impl.MultiPrimaryClient.syncNewState(MultiPrimaryClient.java:299)
at org.mariadb.jdbc.client.impl.MultiPrimaryReplicaClient.setReadOnly(MultiPrimaryReplicaClient.java:294)
at org.mariadb.jdbc.Connection.setReadOnly(Connection.java:309)
```