fix: preserve env when running pytest

Before this patch, pytest used to launch all instances in a clean
environment. It prevented running with `PICODATA_LOG_LEVEL=verbose`.

test/int/conftest.py

@@ -234,7 +234,7 @@ class Instance:
 
         self.process = subprocess.Popen(
             self.command,
-            env=self.env or {},
+            env=self.env or None,
             stdin=subprocess.DEVNULL,
             start_new_session=True,
         )