components: mat-select-trigger is invisible if value is undefined or null

now we have mat-label for this

<mat-form-field>
    <mat-label>Status</mat-label>
    <mat-select placeholder="Please Select">...</mat-select>
</mat-form-field>

@crisbeto I think you can close this issue

@crisbeto, regarding when to show it: I would strongly argue that if the value for ‘trigger’ is not empty, it should always be displayed. The developer has full control over the value provided to the trigger, and with a simple ‘Placeholder will not display unless the triggerValue is empty’ in the docs, it’s clear enough what needs to be done.

If that is not an option, then it should be one, via a flag.

For a use case, I present a multi-select where ‘No Selection’ is allowed. I would like to display ‘All’, ‘Partial ({{count}})’ or ‘None’. I cannot display None, because the selection is empty thus the (empty) placeholder is displayed instead.

I believe that there is similar issue when the select is multiple and no value is selected: the value is then an empty array, evaluating to a falsy value just as null or undefined…

Please let me know if this deserves another issue or not. I wanted to remain on the less-noisy option by default.

@crisbeto while this is currently intended behavior, is there any wisdom in always rendering the trigger value if mat-select-trigger is defined?