optimize class dependencies.

mdrsclient/models/laboratory.py

@@ -1,3 +1,4 @@
+from dataclasses import field
 from typing import Generator
 
 from pydantic.dataclasses import dataclass
@@ -13,7 +14,7 @@ class Laboratory:
 
 @dataclass(frozen=True)
 class Laboratories:
-    items: list[Laboratory]
+    items: list[Laboratory] = field(default_factory=list)
 
     def __iter__(self) -> Generator[Laboratory, None, None]:
         yield from self.items