Fix instances.create to send empty arrays instead of null for volumes parameters

When volumes or existing_volumes parameters are None, send empty arrays []
to the REST API instead of null values. This ensures API compatibility and
proper validation.

Fixes #56

Author: claude

verda/instances/_instances.py

@@ -171,8 +171,8 @@ def create(
             'description': description,
             'location_code': location,
             'os_volume': os_volume,
-            'volumes': volumes,
-            'existing_volumes': existing_volumes,
+            'volumes': volumes if volumes is not None else [],
+            'existing_volumes': existing_volumes if existing_volumes is not None else [],
             'is_spot': is_spot,
             'coupon': coupon,
         }