Don't refer to more content button if always_true is set

If you set `always_show: true` on a trip field and it has more than 50 characters there is a javascript error:

    Error: Missing target element "moreContentButton" for "trix-body" controller

This is because the trix field show component only adds the more content button when `always_show` is `false` but the trix controller always refers to that component without checking if it exists first.

I've updated it so that it checks if that target exists before manipulating it.

app/javascript/js/controllers/trix_body_controller.js

@@ -10,7 +10,7 @@ export default class extends Controller {
   checkContentHeight() {
     const contentHeight = this.contentTarget.scrollHeight
 
-    if (contentHeight > 50) {
+    if (contentHeight > 50 && this.hasMoreContentButtonTarget) {
       this.moreContentButtonTarget.classList.remove('hidden')
     }
   }
@@ -21,7 +21,7 @@ export default class extends Controller {
   }
 
   toggleButtons() {
-    this.moreContentButtonTarget.classList.toggle('hidden')
-    this.lessContentButtonTarget.classList.toggle('hidden')
+    if (this.hasMoreContentButtonTarget) this.moreContentButtonTarget.classList.toggle('hidden')
+    if (this.hasLessContentButtonTarget) this.lessContentButtonTarget.classList.toggle('hidden')
   }
 }