fixup! fix(material-experimental/mdc-typography): strip typography properties with null value

Author: mmalerba

src/material-experimental/mdc-helpers/_mdc-helpers.scss

@@ -116,6 +116,8 @@ $mat-typography-2018-level-mappings: (
           ),
           ()));
 
+  // We need to strip out any keys with a null value. Leaving them in will cause MDC to emit CSS
+  // variables with no fallback value, which breaks some builds.
   $result: ();
   @each $property, $value in $result-with-nulls {
     @if $value != null {