fix(select): display asterisk on label only if empty

[feloy]

Closes #11312

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

If a md-select control has a custom validation on it using $setValidity(false), the required * will show on the chosen value even though the value is populated.

Issue Number: 11312

What is the new behavior?

The required * will show on the value only if no value is chosen. It will still display on placeholder.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[Splaktar]

LGTM, thank you!