[STC-823] Documents not working on exotic browsers

https://community.openproject.org/wp/STC-823

On epiphany, I got the following error:
SyntaxError: The string did not match the expected pattern.
add --- block-note-element.ts:54
BlockNoteElement --- block-note-element.ts:54
define
Module Code --- block-note-element.ts:151

The root cause is that browser_specific_classes in BrowserHelper only adds classes for Chrome, Firefox, Safari, Edge, and mobile/Windows. Epiphany (WebKitGTK on Linux) doesn't match any of those, so it returns []. The ERB template then renders browser-specific-classes="", and "".split(' ') in JS produces [""] — one empty string — which passes the length > 0 guard and then classList.add("") throws in WebKit.

frontend/src/elements/block-note-element.ts

@@ -49,7 +49,7 @@ class BlockNoteElement extends HTMLElement {
     const shadowRoot = this.attachShadow({ mode: 'open' });
 
     this.editorRoot = document.createElement('div');
-    const browserSpecificClasses = this.getAttribute('browser-specific-classes')?.split(' ') ?? [];
+    const browserSpecificClasses = this.getAttribute('browser-specific-classes')?.split(' ').filter(Boolean) ?? [];
     if (browserSpecificClasses.length > 0) {
       this.editorRoot.classList.add(...browserSpecificClasses);
     }

lib/primer/open_project/forms/block_note_editor.html.erb

@@ -41,7 +41,7 @@
       "attachments-collection-key": attachments_collection_key,
       "blocknote-stylesheet-url": blocknote_stylesheet_url,
       "shadow-dom-stylesheet-url": shadow_dom_stylesheet_url,
-      "browser-specific-classes": browser_specific_classes.join(" "),
+      "browser-specific-classes": browser_specific_classes.join(" ").presence,
       "data-test-selector": "blocknote-document-description"
     )
   )