[ISXB-1610] FIX for 1.4X: Impossibility to set InputSystemUIInputModule.localMultiPlayerRoot to null

[hannem-rythmos]

Description

Case ISXB-1610] (https://jira.unity3d.com/browse/ISXB-1610)

This PR removes the null check in MultiplayerEventSystem.InitializePlayerRoot() that prevented setting InputSystemUIInputModule.localMultiPlayerRoot to null. We found that the check was introduced in PR #1547, but no specific reason was provided

Since localMultiPlayerRoot is already safely used (e.g., in IsMoveAllowed() via its own null check), we have removed the null check.

Testing status & QA

Added a UnityTest to validate that setting InputSystemUIInputModule.localMultiPlayerRoot to null allows full UI navigation as expected.

Overall Product Risks

Please rate the potential complexity and halo effect from low to high for the reviewers. Note down potential risks to specific Editor branches if any.

• Complexity: low
• Halo Effect: low

Comments to reviewers

As localMultiPlayerRoot is having its own null check, we have removed the PlayerRoot null check in InitializePlayerRoot().

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[unity-cla-assistant]

All committers have signed the CLA.

[hannem-rythmos]

Closing this PR as we have raised PR in Input system repo. #2222