Fix avatar caching for non-user rendering

[skjnldsv]

• Do not cache if the user is not defined
• Added debug level
• Fallback to letters if the url is invalid

<Avatar
	:disable-menu="true"
	:disable-tooltip="true"
	display-name="Hello User"
	:is-no-user="true"
	:size="40"
	url="http://this.is.a.fakeUrl.jpg"
	class="app-content-list-item-icon" />

[PVince81]

is that for guest users or another scenario ? (steps to reproduce would be nice for verification)

[PVince81]

👍 tested, makes sense