fixing issue - No config in tenant-mgt.xml to read the value of OrganizationalSubContextAttribute - CARBON-14863

You can rearrange the code as follows, which improves the readability and less method calls.

String userContextValue = tenantMgtConfig.getTenantStoreProperties().get(UserCoreConstants.TenantMgtConfig.PROPERTY_ORG_SUB_CONTEXT_USER_CONTEXT_VALUE);
if (userContextValue == null) {
    userContextValue = LDAPConstants.USER_CONTEXT_NAME;
}
String userContextRDN = orgSubContextAttribute + "=" + userContextValue;