fixup! refactor(material/theming): clean up theme file and use mdc typography everywhere

Author: mmalerba

src/material/core/typography/_typography.scss

@@ -87,6 +87,9 @@
 // Components using this function should be migrated to normalize to the 2018 style config instead.
 // New components should not use this function.
 @function mat-private-typography-to-2014-config($config) {
+  @if $config == null {
+    @return null
+  }
   @if not mat-private-typography-is-2014-config($config) {
     @return mat-typography-config(
         $display-4: map-get($config, headline-1),
@@ -111,6 +114,9 @@
 // 2014 - https://material.io/archive/guidelines/style/typography.html#typography-styles
 // 2018 - https://material.io/design/typography/the-type-system.html#type-scale
 @function mat-private-typography-to-2018-config($config) {
+  @if $config == null {
+    @return null
+  }
   @if mat-private-typography-is-2014-config($config) {
     @return (
         headline-1: map-get($config, display-4),