feat(MatAutocompleteTrigger): support null type for autocompleteAttribute input

[atzork]

Feature Description

Change the type of the autocompleteAttribute input parameter in MatAutocompleteTrigger from string to string | null.

This will allow removing the HTML autocomplete attribute from DOM element when necessary.

Use Case

No response

[crisbeto]

It's not a problem to add it, but I wonder what is the use case? Currently we default it to off which should be identical to it not being there at all.