Add empty state message for follow recommendations in web UI (#16161)

app/javascript/mastodon/features/follow_recommendations/index.js

@@ -75,10 +75,14 @@ class FollowRecommendations extends ImmutablePureComponent {
 
           {!isLoading && (
             <React.Fragment>
-              <div>
-                {suggestions.map(suggestion => (
+              <div className='column-list'>
+                {suggestions.size > 0 ? suggestions.map(suggestion => (
                   <Account key={suggestion.get('account')} id={suggestion.get('account')} />
-                ))}
+                )) : (
+                  <div className='column-list__empty-message'>
+                    <FormattedMessage id='empty_column.follow_recommendations' defaultMessage='Looks like no suggestions could be generated for you. You can try using search to look for people you might know or explore trending hashtags.' />
+                  </div>
+                )}
               </div>
 
               <div className='column-actions'>

app/javascript/styles/mastodon/components.scss

@@ -2525,6 +2525,22 @@ a.account__display-name {
   }
 }
 
+.column-list {
+  margin: 0 20px;
+  border: 1px solid lighten($ui-base-color, 8%);
+  background: darken($ui-base-color, 2%);
+  border-radius: 4px;
+
+  &__empty-message {
+    padding: 40px;
+    text-align: center;
+    font-size: 16px;
+    line-height: 24px;
+    font-weight: 400;
+    color: $darker-text-color;
+  }
+}
+
 .compose-panel {
   width: 285px;
   margin-top: 10px;