Backport OPENDJ-6217: NPE when running supportextract tool on upgraded instance

Found with 7.0.0-SNAPSHOT rev. 6fbc8ef8957 .

With our automated test we hit NullPointerException, when running a supportextract tool to collect data on upgraded instance (5.5.0=>7.0.0). We hit this issue when upgrading from version < 6.0.0. I tried also upgrade from 5.5.0 to 6.5.1 and I didn't hit the issue.

Steps to reproduce:

1. Install DS-5.5.0
2. Do the upgrade to 7.0.0
3. Run supportextract tool

supportextract -D "cn=Directory Manager" -w "password" -d "/tmp" 
...
The following archive has been created :
/tmp/opendj-support-data-20190417-102702.zip

Current behavior:
We have following NPE and messages on stderr.

null
Continuing data gathering though there was an error: Could not collect all the
files: java.lang.NullPointerException
There was at least one error during the tasks execution: 
Could not collect all the files: java.lang.NullPointerException

Expected behavior:
Do not hit the NPE and have more detailed message, if there is an issue. It is not obvious why the tool can't collect all files.

It is possible to reproduce the issue with automated test:

python3 run-pybot.py --cfg example.cfg -s clu_group.supportExtract -t default -v DJ

Be sure to put attached example.cfg to /path/to/framework/config/ and change the default hostname and java path according to your workspace settings.