fix(select): form remains valid after empty option is selected

- change md-select's `$isEmpty()` implementation from a naive falsy check
  to a more robust check that is similar to AngularJS' default
- add empty option to validations demo

Fixes #10005

Author: Splaktar

src/components/select/demoValidations/index.html

@@ -14,6 +14,7 @@
       <md-input-container flex="50">
         <label>Favorite Color</label>
         <md-select name="favoriteColor" ng-model="favoriteColor" required>
+          <md-option value=""></md-option>
           <md-option value="red">Red</md-option>
           <md-option value="blue">Blue</md-option>
           <md-option value="green">Green</md-option>

src/components/select/select.js

@@ -589,8 +589,8 @@ function SelectDirective($mdSelect, $mdUtil, $mdConstant, $mdTheming, $mdAria, $
 
       function inputCheckValue() {
         // The select counts as having a value if one or more options are selected,
-        // or if the input's validity state says it has bad input (eg string in a number input)
-        // we must do this on nextTick as the $render is sometimes invoked on nextTick.
+        // or if the input's validity state says it has bad input (eg: string in a number input).
+        // We must do this on nextTick as the $render is sometimes invoked on nextTick.
         $mdUtil.nextTick(function () {
           containerCtrl && containerCtrl.setHasValue(
             selectMenuCtrl.getSelectedLabels().length > 0 || (element[0].validity || {}).badInput);
@@ -850,7 +850,10 @@ function SelectMenuDirective($parse, $mdUtil, $mdConstant, $mdTheming) {
       self.ngModel.$isEmpty = function($viewValue) {
         // We have to transform the viewValue into the hashKey, because otherwise the
         // OptionCtrl may not exist. Developers may have specified a trackBy function.
-        return !self.options[self.hashGetter($viewValue)];
+        var hashedValue = self.options[self.hashGetter($viewValue)] ? self.options[self.hashGetter($viewValue)].value : null;
+        // Base this check on the default AngularJS $isEmpty() function.
+        // eslint-disable-next-line no-self-compare
+        return !angular.isDefined(hashedValue) || hashedValue === null || hashedValue === '' || hashedValue !== hashedValue;
       };
 
       // Allow users to provide `ng-model="foo" ng-model-options="{trackBy: '$value.id'}"` so