fix(material/tabs): prevent tab header from collapsing when empty inside a drop list

Adds a `min-height` to the tab header when it is placed inside a drop list. This is useful when dragging between connected lists since the header may collapse when it has no tabs.

Author: crisbeto

src/material/tabs/_tabs-common.scss

@@ -393,6 +393,18 @@ $mat-tab-animation-duration: 500ms !default;
   [mat-align-tabs='end'] > #{$parent} & {
     justify-content: flex-end;
   }
+
+  // Prevent the header from collapsing when it is a drop list. This is useful,
+  // because its height may become zero once all the tabs are dragged out.
+  .cdk-drop-list &,
+  &.cdk-drop-list {
+    @include token-utils.use-tokens(
+      tokens-mdc-secondary-navigation-tab.$prefix,
+      tokens-mdc-secondary-navigation-tab.get-token-slots()
+    ) {
+      @include token-utils.create-token-slot(min-height, container-height);
+    }
+  }
 }
 
 // Structural styles for the element that wraps the paginated container's content.