Fix bad width w/no options in multi-select DAG parameter (#51516)

Address inproper input element width when no options are present in a
multi-select field for a DAG parameter in the DAG trigger page.  If the
parameter:

* is in a section: the element would become too narrow making the
  dropdown options unreadable.
* is NOT in a section: the element would overflow it's parent if
  re-sized smaller

Address by setting width to fill the parent element as other DAG trigger
paramter input fields behave.

closes: #50322

Author: fredthomsen

airflow/www/static/js/trigger.js

@@ -170,6 +170,7 @@ function initForm() {
           $(elementId).select2({
             placeholder: "Select Values",
             allowClear: true,
+            width: "100%",
           });
           elements[i].addEventListener("blur", updateJSONconf);
         } else if (elements[i].type === "checkbox") {