directory pruning plugin does not process all entries

Issue Summary

Directory pruning plugin does not deactivate all users. This happens if some users have empty/null external id.

Steps to Reproduce

1. Setup delegated directory
2. In the Crowd database set external id of the first alphabetical user to null

Expected Results

All users that are inactive in the remote directory should be deactivated.

Actual Results

Only users that are in the top 1000 (alphabetically) will be deactivated.

Workaround

There is no good workaround for that.

One option would be to set some random external ids for users with empty/null external ids. Obviously it is not ideal to have random external ids in the database.