TRUNK-6219: Obs should be considered a group if it ever had group members

[icrc-loliveira]

Description of what I changed

On O3 if you try to void an obsGroup all is Obs it will fail with:

• ['Obs #1551302' failed to validate with reason: error.noValue, valueText: Cannot be empty or null]
  This is due to the fact that all in Obs inside the obsGroup have already been voided and does not recognize it as an obsGroup.

Note: Please add this development to version 2.5.x

Issue I worked on

see https://openmrs.atlassian.net/browse/TRUNK-6219

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.

  No? Unsure? -> configure your IDE, format the code and add the changes with git add . && git commit --amend

• I have added tests to cover my changes. (If you refactored
  existing code that was well tested you do not have to add tests)

  No? -> write tests and add them to this commit git add . && git commit --amend

• I ran mvn clean package right before creating this pull request and
  added all formatting changes to my commit.

  No? -> execute above command

• All new and existing tests passed.

  No? -> figure out why and add the fix to your commit. It is your responsibility to make sure your code works.

• My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master

[icrc-loliveira]

Hello @ibacher, Please review this PR, and if everything is OK port it to version 2.5.x