fix(select): display asterisk on label only if empty (#11355)

Closes #11312

Author: feloy

src/components/select/select.scss

@@ -105,7 +105,7 @@ md-select {
   margin: 2.5*$baseline-grid 0 3*$baseline-grid + 2 0;
 
   &[required], &.ng-required {
-    &.ng-invalid:not(.md-no-asterisk) {
+    &.ng-empty.ng-invalid:not(.md-no-asterisk) {
       .md-select-value span:first-child:after {
         content: ' *';
         font-size: 13px;