refactor(material/snack-bar): fix out-of-sync code (#25302)

Fixes some code that is out of sync with the code we have internally. I added the extra `@if` when I was resolving issues with #25273.

Author: crisbeto

src/material-experimental/mdc-snack-bar/_snack-bar-theme.scss

@@ -34,13 +34,15 @@
   $config: mat.private-typography-to-2018-config(
       mat.get-typography-config($config-or-theme));
   @include mat.private-using-mdc-typography($config) {
-    .mat-mdc-snack-bar-container {
-      @include mdc-snackbar-theme.theme((
-        supporting-text-font: mat.font-family($config, body-2),
-        supporting-text-line-height: mat.line-height($config, body-2),
-        supporting-text-size: mat.font-size($config, body-2),
-        supporting-text-weight: mat.font-weight($config, body-2),
-      ));
+    @if ($config) {
+      .mat-mdc-snack-bar-container {
+        @include mdc-snackbar-theme.theme((
+          supporting-text-font: mat.font-family($config, body-2),
+          supporting-text-line-height: mat.line-height($config, body-2),
+          supporting-text-size: mat.font-size($config, body-2),
+          supporting-text-weight: mat.font-weight($config, body-2),
+        ));
+      }
     }
   }
 }