FIX: wait for the end of search requests (#26240)

Prior to this fix, if you were following this series of events:
- type something in a select-kit filter with async search
- query starts
- type something again
- first query finished with no results
- second query starts
- 💥 we would show a no content found for a split second
- second query finishes
- we display a list of results

app/assets/javascripts/select-kit/addon/components/multi-select.hbs

@@ -42,7 +42,7 @@
     {{/each}}
 
     {{#if this.selectKit.filter}}
-      {{#if this.selectKit.hasNoContent}}
+      {{#if (and this.selectKit.hasNoContent (not this.selectKit.isLoading))}}
         <span class="no-content" role="alert">
           {{i18n "select_kit.no_content"}}
         </span>

app/assets/javascripts/select-kit/addon/components/single-select.hbs

@@ -28,7 +28,7 @@
     {{/each}}
 
     {{#if this.selectKit.filter}}
-      {{#if this.selectKit.hasNoContent}}
+      {{#if (and this.selectKit.hasNoContent (not this.selectKit.isLoading))}}
         <span class="no-content" role="alert">
           {{i18n "select_kit.no_content"}}
         </span>