artifacts: omit blueprint when value is None

When a blueprintId is not supplied, the key should be omitted from the json passed as the request
body.

Signed-off-by: Mike Szczys <mike@golioth.io>

Author: szczys

golioth/golioth.py

@@ -780,9 +780,11 @@ async def upload(self,
             'content': b64encode(path.open('rb').read()).decode(),
             'version': version,
             'package': package,
-            'blueprintId': blueprint_id
         }
 
+        if blueprint_id is not None:
+            json['blueprintId'] = blueprint_id
+
         try:
             response = await self.project.client.post('artifacts', json=json)
         except httpx.HTTPStatusError as err: