Fix yaml-editor for new documents (again)

The check for the querySelector chain should be that we found a
textArea, not that we found a textArea and it has a truthy value.

The empty string is falsey in JavaScript land, so checking !value
incorrectly meant we got the "DOM structure did not match" warning.

app/assets/javascripts/components/yaml-editor.js

@@ -35,13 +35,13 @@ window.GOVUK.Modules = window.GOVUK.Modules || {}
       '.app-c-govspeak-editor__preview-button-wrapper'
     )
     const textArea = innerContainer?.querySelector('#edition_body')
-    const value = textArea?.value
-    if (!value) {
+    if (!textArea) {
       console.warn(
         'YamlEditor: DOM structure did not match expectations, falling back to doing nothing'
       )
       return
     }
+    const value = textArea.value
 
     const monacoHostDiv = document.createElement('div')
     monacoHostDiv.style.width = '100%'