fix(select): floating label hidden w/ placeholder and no value

[Splaktar]

PR Checklist

Please check 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?

• floating label is not displayed when placeholder is specified and no selection has been made in a md-input-container

Fixes #10116

What is the new behavior?

• don't hide the floating label in an md-input-container with a md-select
  that has a placeholder attribute specified

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Given the following:

        <md-input-container>
          <label>State</label>
          <md-select placeholder="Please select" ng-model="ctrl.userState">
            <md-option><em>None</em></md-option>
            <md-option ng-repeat="state in ctrl.states" ng-value="state.abbrev">
              {{state.abbrev}}
            </md-option>
          </md-select>
        </md-input-container>

Before

After

[mmalerba]

LGTM