improve attribute handling

packages/core/src/types.ts

@@ -7,8 +7,8 @@ export type Extensions = (Extension | NodeExtension | MarkExtension)[]
 export type Attribute = {
   default: any,
   rendered?: boolean,
-  renderHTML?: ((attributes: { [key: string]: any }) => any) | null,
-  parseHTML?: ((element: HTMLElement) => { [key: string]: any }) | null,
+  renderHTML?: ((attributes: { [key: string]: any }) => { [key: string]: any } | null) | null,
+  parseHTML?: ((element: HTMLElement) => { [key: string]: any } | null) | null,
 }
 
 export type Attributes = {

packages/core/src/utils/getRenderedAttributes.ts

@@ -12,7 +12,7 @@ export default function getRenderedAttributes(nodeOrMark: Node | Mark, extension
         }
       }
 
-      return item.attribute.renderHTML(nodeOrMark.attrs)
+      return item.attribute.renderHTML(nodeOrMark.attrs) || {}
     })
     .reduce((attributes, attribute) => {
       return mergeAttributes(attributes, attribute)

packages/core/src/utils/injectExtensionAttributesToParseRule.ts

@@ -27,7 +27,7 @@ export default function injectExtensionAttributesToParseRule(parseRule: ParseRul
         .filter(item => item.attribute.rendered)
         .reduce((items, item) => {
           const attributes = item.attribute.parseHTML
-            ? item.attribute.parseHTML(node as HTMLElement)
+            ? item.attribute.parseHTML(node as HTMLElement) || {}
             : {
               [item.name]: (node as HTMLElement).getAttribute(item.name),
             }