mat-select placeholder not working.

[KissBalazs]

🐞 bug report

Affected Package

Angular Material 9

Is this a regression?

Not sure.

Description

Using placeholder attribute has no effect in mat-select, if mat-label is provided also.

🔬 Minimal Reproduction

Forked from the mat-select example in the documentation:

https://stackblitz.com/edit/angular-qlpnim-vyfruq?file=src/app/select-overview-example.html

<h4>Basic mat-select</h4>
<mat-form-field>
  <mat-label>Favorite food</mat-label>
  <mat-select placeholder="No value selected">  <!-- this is not showing-->
    <mat-option *ngFor="let food of foods" [value]="food.value">
      {{food.viewValue}}
    </mat-option>
  </mat-select>
</mat-form-field>

Expected and Actual behavior.
Expected: If provided, the label floats to the top of the field, and the placeholder text can be seen if no value is selected.
Actual: the label acts as a placeholder.

🔥 Exception or Error

--

🌍 Your Environment

Angular Version:

@angular/material                  9.0.1

Anything else relevant?

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