Add is-empty modifier to file upload (#1383)

* add classes

* add to changelog

sass/elements/form.sass

@@ -269,6 +269,11 @@ $help-size: $size-small !default
     .file-name
       border-bottom-left-radius: 0
       border-top-left-radius: 0
+    &.is-empty
+      .file-cta
+        border-radius: $file-radius
+      .file-name
+        display: none
   &.is-centered
     justify-content: center
   &.is-right