fix(material/autocomplete): don't assign to model value while typing when requireSelection is enabled

[crisbeto]

Follow-up to #27423 based on the feedback. Usually mat-autocomplete assigns to the model as the user is typing which may not be desired when requireSelection is enabled, because at the end of the selection either an option value will set or it'll be reset.

These changes add a condition so that the value isn't assigned while typing and requireSelection is enabled.

[crisbeto]

I had to add and remove the merge label to verify that the change will be merged in the right branch.

[mozgor]

Regarding your early comments in previous thread and especially this one, this fix means that despite being against the generic expectations of not interacting directly with the DOM while dealing with reactive forms (quoting you) this is the actual way of filtering autocomplete options ? I understand the reasoning behind this fix, but I'd double check before propagating updated guidelines inside our project.

[crisbeto]

Yes, I had to make the tradeoff because it would've been a weird experience if the form control was emitting events with the string value while the user is typing and then a null event at the end if they don't select something since there's no way to distinguish where the value is coming from.

[mozgor]

Make sense, even if it would have been easier for my use case to filter out null value rather than switching source. Controls "valuesChanges" stream was very convenient especially to chain filter / debounce operators.

I'll figure that out, thanks for the confirmation.

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