fix: No value supplied for attribute src

It shouldn't be possible to have an image node with no src, but it does happen occassionally and this prevents the document from persisting. It's better to have a broken image than a doc that won't save

shared/editor/nodes/Image.tsx

@@ -168,7 +168,9 @@ export default class Image extends Node {
     return {
       inline: true,
       attrs: {
-        src: {},
+        src: {
+          default: "",
+        },
         width: {
           default: undefined,
         },