code: @Nullable
      public MongoCredential getCredential() {
          if (getCredentialsList().size() > 1) {
              throw new IllegalStateException("Instance constructed with more than one MongoCredential");
          } else if (getCredentialsList().isEmpty()) {
              return null;
          } else {
              return getCredentialsList().get(0);
          }
      }
docs: Gets the credential that this client authenticates all connections with
  
  @return the credential, which may be null in unsecured deployments
  @since 3.9