fix: Update avatarType prop to be optional

[vthomas13]

Description

Fixing AvatarGroup tsx test failures due to avatarType being a required type

Related issues

Fixes: #23080

Manual testing steps

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.60%. Comparing base (cb36096) to head (30dde9a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #23184   +/-   ##
========================================
  Coverage    68.60%   68.60%           
========================================
  Files         1096     1096           
  Lines        43262    43262           
  Branches     11528    11528           
========================================
  Hits         29679    29679           
  Misses       13583    13583           

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  • common: 0, 1, 2, 3, 4, 5, 6
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Page Load Metrics (1265 ± 424 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	83	201	127	33	16
		domContentLoaded	10	79	33	19	9
		load	69	2474	1265	883	424
		domInteractive	10	79	33	19	9

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 0 Bytes (0.00%)
• common: 0 Bytes (0.00%)