Always hide remove button if no value is selected (#744)

In some cases, the remove item X button is not hidden when there is no
selection made from the dropdown. This is not as usable, since there's
no item to clear, hence an effective noop.

The condition I saw seeing this issue arise is in single selects with
a placeholder value, a selection has been made, and then cleared: the
button HTML remains.

To fix, add some styles to hide the button if the select doesn't have a
value selected, in this case keying off of it's parents `data-value`
property.

fixes #684

public/assets/styles/choices.css

@@ -72,6 +72,10 @@
   box-shadow: 0px 0px 0px 2px #00bcd4;
 }
 
+.choices[data-type*='select-one'] .choices__item[data-value=''] .choices__button {
+  display: none;
+}
+
 .choices[data-type*='select-one']:after {
   content: '';
   height: 0;

src/styles/choices.scss

@@ -83,6 +83,9 @@ $choices-icon-cross-inverse: url(data:image/svg+xml;base64,PHN2ZyB3aWR0aD0iMjEiI
       box-shadow: 0px 0px 0px 2px $choices-highlight-color;
     }
   }
+  .#{$choices-selector}__item[data-value=''] .#{$choices-selector}__button {
+    display: none;
+  }
   &:after {
     content: '';
     height: 0;