fix: correct new_artifact methods signature (#503)

Artifact class has nullable description with default None
`Artifact(description: str | None = None)`

But, utilities functions creates and artifact with empty description by
default `description=''`

Release-As: 0.3.10

Author: Lancetnik

src/a2a/utils/artifact.py

@@ -9,7 +9,9 @@
 
 
 def new_artifact(
-    parts: list[Part], name: str, description: str = ''
+    parts: list[Part],
+    name: str,
+    description: str | None = None,
 ) -> Artifact:
     """Creates a new Artifact object.
 
@@ -32,7 +34,7 @@ def new_artifact(
 def new_text_artifact(
     name: str,
     text: str,
-    description: str = '',
+    description: str | None = None,
 ) -> Artifact:
     """Creates a new Artifact object containing only a single TextPart.
 
@@ -54,7 +56,7 @@ def new_text_artifact(
 def new_data_artifact(
     name: str,
     data: dict[str, Any],
-    description: str = '',
+    description: str | None = None,
 ) -> Artifact:
     """Creates a new Artifact object containing only a single DataPart.
 

tests/utils/test_artifact.py

@@ -38,7 +38,7 @@ def test_new_artifact_empty_description_if_not_provided(self):
         parts = [Part(root=TextPart(text='Another sample'))]
         name = 'Artifact_No_Desc'
         artifact = new_artifact(parts=parts, name=name)
-        self.assertEqual(artifact.description, '')
+        self.assertEqual(artifact.description, None)
 
     def test_new_text_artifact_creates_single_text_part(self):
         text = 'This is a text artifact.'