Add :empty to .custom-file-control selector (#22176)

<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control"></span></label>
VS
<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control">test.txt</span></label>

scss/_custom-forms.scss

@@ -229,7 +229,7 @@
   @include box-shadow($custom-file-box-shadow);
 
   @each $lang, $text in map-get($custom-file-text, placeholder) {
-    &:lang(#{$lang})::after {
+    &:lang(#{$lang}):empty::after {
       content: $text;
     }
   }