I think the better solution to add a check in snippet which looks for the avatar, and if empty it uses the default one.
Or if the avatar field is a TV, no need for a snippet.
You can set the default value directly in the TV itself. Or use PHx with an ifempty or if... then...else clause which allow a different output if the field is empty = default avatar, if the field has a value = avatar field.
I’ve done it several times, works like a charm
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