Fix sending of emptystring class for Prosemirror decoration (#1004)

Prosemirror does not handle emptystring classes well for decorations when the decoration applies to a range that has other decorations. For example, both a spellchecking plugin and the tiptap highlighting plugin.

Furthermore, there is no reason to emit empty decorations, so this change resolves that issue as well.

packages/tiptap-extensions/src/plugins/Highlight.js

@@ -47,6 +47,11 @@ function getDecorations({ doc, name }) {
         }
       })
       .forEach(node => {
+        if (node.classes.length === 0) {
+          // Do not emit empty decorations.
+          return
+        }
+
         const decoration = Decoration.inline(node.from, node.to, {
           class: node.classes.join(' '),
         })