Initialize children in Document.__init__() (#209)

* Initialize children in Document.__init__()

* Remove redundant call to super().__init__()

Author: apetresc

marko/block.py

@@ -78,7 +78,7 @@ class Document(BlockElement):
     virtual = True
 
     def __init__(self) -> None:
-        super().__init__()
+        self.children = []
         self.link_ref_defs: dict[str, tuple[str, str]] = {}