fix(rtl): do not emit if a left/right value or replacement is null

PiperOrigin-RevId: 370624600

packages/mdc-rtl/_rtl.scss

@@ -292,14 +292,30 @@ $include: true !default;
     @if $left-replace == null and $right-replace == null {
       @error 'mdc-rtl: $replace may only be used with strings but neither left nor right values are strings.';
     }
+
+    // If any replacements are null, treat the entire value as null (do not
+    // emit anything).
+    @each $name, $replacement in $replace {
+      @if $replacement == null {
+        $left-value: null;
+        $right-value: null;
+      }
+    }
   }
 
-  @include _property($left-property, $left-value, $replace: $left-replace);
-  @include _property($right-property, $right-value, $replace: $right-replace);
+  // Do not emit if either value are null
+  @if $left-value and $right-value {
+    @include _property($left-property, $left-value, $replace: $left-replace);
+    @include _property($right-property, $right-value, $replace: $right-replace);
 
-  @include rtl {
-    @include _property($left-property, $right-value, $replace: $right-replace);
-    @include _property($right-property, $left-value, $replace: $left-replace);
+    @include rtl {
+      @include _property(
+        $left-property,
+        $right-value,
+        $replace: $right-replace
+      );
+      @include _property($right-property, $left-value, $replace: $left-replace);
+    }
   }
 }