Derive a block directive's empty content from its node, and let readEmptyKeys own the attrs=empty-stands-alone rule

src/markdown/empty-keys.ts

@@ -16,7 +16,8 @@ export function spellsEmpty(value: DirectiveValue | undefined): boolean {
   return value?.spelling === emptyValue
 }
 
-export function readEmptyKeys(attributes: DirectiveAttributes, keys: readonly EmptyKey[]): Read<EmptyKeysRead> {
+// `held` is whether the directive spells an attribute outside {attrs}: an argument or a content slot.
+export function readEmptyKeys(type: string, attributes: DirectiveAttributes, keys: readonly EmptyKey[], held: boolean): Read<EmptyKeysRead> {
   const rest = new Map(attributes)
   const empty = new Set<EmptyKey>()
   for (const key of keys) {
@@ -26,5 +27,6 @@ export function readEmptyKeys(attributes: DirectiveAttributes, keys: readonly Em
     empty.add(key)
     rest.delete(key)
   }
+  if (empty.has('attrs') && (held || [...rest.keys()].some((key) => key !== 'marks'))) return { fault: unsupportedNodeShape(`${type} spells attrs=empty beside an attribute it holds`) }
   return { value: { empty, rest } }
 }