fix(material/tabs): prevent tab header from collapsing when empty

Adds a `min-height` to the header so it doesn't collapse when there are no tabs. This is useful when the tabs have drag&drop enabled.

Author: crisbeto

src/material/tabs/_tabs-common.scss

@@ -393,6 +393,15 @@ $mat-tab-animation-duration: 500ms !default;
   [mat-align-tabs='end'] > #{$parent} & {
     justify-content: flex-end;
   }
+
+  @include token-utils.use-tokens(
+    tokens-mdc-secondary-navigation-tab.$prefix,
+    tokens-mdc-secondary-navigation-tab.get-token-slots()
+  ) {
+    // Prevent the header from collapsing when all the items are removed. This is useful
+    // if the tabs are set up with drag&drop and one of the lists is emptied out.
+    @include token-utils.create-token-slot(min-height, container-height);
+  }
 }
 
 // Structural styles for the element that wraps the paginated container's content.