CMCL-1574: zero member targetgroup behaves better

[glabute]

Purpose of this PR

CMCL-1574:
When all members of a target group are disabled, target group should retain its most recent valid position and dimensions, to avoid camera pops.

This fixes a regression reported in CMCL-1574

Testing status

• Added an automated test
• Passed all automated tests
• Manually tested

Documentation status

• Updated CHANGELOG
• Updated README (if applicable)
• Commented all public classes, properties, and methods
• Updated user documentation

Technical risk

low

[AntoineCharton]

Suggested change

	## [Unreeleased] - 2024-01-02
	## [2.11.0] - 2024-01-02

To fix the tests

[glabute]

Thanks, done

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 26.55%. Comparing base (e3c265f) to head (434d774).
Report is 1 commits behind head on release/2.10.

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Additional details and impacted files

@@              Coverage Diff              @@
##           release/2.10     #954   +/-   ##
=============================================
  Coverage         26.54%   26.55%           
=============================================
  Files               126      126           
  Lines             18893    18889    -4     
=============================================
  Hits               5016     5016           
+ Misses            13877    13873    -4     

Files	Coverage Δ
...chine/Runtime/Behaviours/CinemachineTargetGroup.cs	0.00% <0.00%> (ø)

[AntoineCharton]

Bug is fixed.