Handle textbox without existing aria-described-by

jQuery.attr returns `undefined` if an element does not have an
attribute. We want an empty string, rather than the default of coercing
`undefined` to the string `'undefined'`.

app/assets/javascripts/updateStatus.js

@@ -48,7 +48,14 @@
         .attr('id', id);
 
       this.$textbox
-        .attr('aria-described-by', this.$textbox.attr('aria-described-by') + ' ' + id)
+        .attr(
+          'aria-described-by',
+          (
+            this.$textbox.attr('aria-described-by') || ''
+          ) + (
+            this.$textbox.attr('aria-described-by') ? ' ' : ''
+          ) + id
+        )
         .on('input', throttle(this.update, 150))
         .trigger('input');