Make MarkdownParser.parse return type non-nullable

FIX: Fix the type of `MarkdownParser.parse` to be non-nullable.

src/from_markdown.ts

@@ -230,7 +230,7 @@ export class MarkdownParser {
     let state = new MarkdownParseState(this.schema, this.tokenHandlers), doc
     state.parseTokens(this.tokenizer.parse(text, markdownEnv))
     do { doc = state.closeNode() } while (state.stack.length)
-    return doc || this.schema.topNodeType.createAndFill()
+    return doc || this.schema.topNodeType.createAndFill()!
   }
 }