fix(material/tabs): prevent header from collapsing if there are no tabs

Fixes that the tab header was collapsing when it doesn't have any tabs which can be problematic when implementing drag&drop in it.

Author: crisbeto

src/material/tabs/_tabs-common.scss

@@ -383,6 +383,15 @@ $mat-tab-animation-duration: 500ms !default;
   display: flex;
   flex: 1 0 auto;
 
+  @include token-utils.use-tokens(
+    tokens-mdc-secondary-navigation-tab.$prefix,
+    tokens-mdc-secondary-navigation-tab.get-token-slots()
+  ) {
+    // Prevents the container from collapsing if there are no tabs
+    // which can be problematic for tab headers that use drag&drop.
+    @include token-utils.create-token-slot(min-height, container-height);
+  }
+
   // We need to set the parent here explicitly, in order to prevent the alignment
   // from any parent tab groups from propagating down to the children when nesting.
   // Note that these are used as inputs so they shouldn't be changed to `mat-mdc-`.