Bugfix NPE that occurs during version upgrade with empty/default resource isolation group

[ctbrennan]

Why I'm doing:

When upgrading for the first time to use resource isolation groups, there's an issue where the TabletComputeNodeMapper doesn't know about the default/empty resource isolation group, due to the way that the modifyComputeNode function worked.

What I'm doing:

• modifyComputeNode is idempotent, so we'll let it run regardless of whether the old and new resource isolation groups match.
• Also, improve situations where a resource isolation group has no CN, by returning empty collections instead of null, which reduces chances for NPE.
• Also expand parsing of resource isolation group in statements to allow unsetting/setting-to-empty the resource isolation group.

Fixes #issue https://jira.pinadmin.com/browse/RTA-6269

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• [] Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.3
  • 3.2
  • 3.1
  • 3.0
  • 2.5