bug(mat-form-field): updateOutlineGap does not recalculate when label is changed to be empty

[ko-tori]

Description

Some logic in updateOutlineGap skips recalculation when the label content is falsy. This prevents the outlineGap from updating when the label is cleared. It seems like the same logic also prevents the outline gap from updating when the label is removed altogether. In these cases, instead of just returning, the outline should be restored to default.

Reproduction

https://stackblitz.com/edit/angular-nmdjyj

Steps to reproduce:

1. Have a form-field with appearance="outline" and a mat-label
2. After loading and outline gap is calculated, set the contents of the mat-label to ""

Expected Behavior

Focusing the form-field should show no outline gap

Actual Behavior

Focusing the form-field should shows an outline gap

Environment

• Angular:
• CDK/Material:
• Browser(s):
• Operating System (e.g. Windows, macOS, Ubuntu):

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