MatAutocompleteTrigger.panelClosingActions emmiting even if the panel isn't being closed

[julianobrasil]

Bug, feature request, or proposal:

Bug?

What is the expected behavior?

MatAutocompleteTrigger.panelClosingActions should emmit only upon a closing panel action

What is the current behavior?

It's emmiting a void when the input is focused and the user hit tab key to leave it, even if the panel is already closed.

What are the steps to reproduce?

https://stackblitz.com/edit/autocomplete-force-selection

Open the Developer Tools, select the input and hit tab. It will emmit an undefined.

Is there anything else we should know?

I'm not sure if it's covered by #8533 (haven't taken a look at the code)

cc @crisbeto

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