Backport OPENIDM-6291: '/_id: Expecting a value' warning when adding a Role with an On Assignment script

When assigning a Role to a user, if the Role references an Assignment which contains a basic On Assignment script then this will throw an "/_id: Expecting a value" warning message.

To reproduce the problem behaviour:

1). Start OpenIDM 4.5 with sample2b and configure this for an OpenDJ.

2). Configure an Assignment 'testassignment' (via Manage > Assignment and note that it should be managedUser_systemLdapAccounts in the mapping) that updates an LDAP attribute, eg. Description.

3). Create a Role 'testrole' (referencing the Assignment) and then assign this role to a user (via the User page not the Roles page), as a provisioning role, and check that the attribute value has been updated in LDAP to verify the assignment functionality.

4). Go to Assignment > Events > On Assignment and set to Inline Script and simply add the following:

logger.info("test onassign");

5). Assign the role to a different user. The role will show as assigned however the following will be seen in the logs:

-> Unexpected failure during source synchronization
org.forgerock.openidm.sync.impl.SynchronizationException: /_id: Expecting a value
    at org.forgerock.openidm.sync.impl.ObjectMapping$SyncOperation.performAction(ObjectMapping.java:1915)
    at org.forgerock.openidm.sync.impl.ObjectMapping$SourceSyncOperation.sync(ObjectMapping.java:2225)
<SNIP>

Expected behaviour:

The "/_id: Expecting a value" message should not occur.