vdoc: fix markdown and plaintext rendering (#6882)

vlib/v/doc/doc.v

@@ -296,6 +296,9 @@ pub fn (mut d Doc) file_ast(file_ast ast.File) map[string]DocNode {
 		}
 	}
 	d.fmt.mod2alias = map[string]string{}
+	if contents[''].kind != .const_group {
+		contents.delete('')
+	}
 	return contents
 }
 

vlib/v/doc/doc_test.v

@@ -13,6 +13,6 @@ fn test_generate_from_mod() {
 	}
 	assert nested_mod_doc.head.name == nested_mod_name
 	assert nested_mod_doc.head.content == 'module $nested_mod_name'
-	assert nested_mod_doc.contents.len == 4
+	assert nested_mod_doc.contents.len == 3
 	assert nested_mod_doc.contents['ChunkScanner'].children.len == 3
 }