style: explicit assert on noop test (TQ001 zero-assert budget)

BerriAI/litellm CI's test-quality budget flagged
test_raise_if_missing_package_is_noop_when_present as a zero-assert
test (TQ001). Make the intent explicit: raise_if_missing_package()
must return None when the package IS installed.

tests/test_litellm/proxy/guardrails/test_conduct_guardrail.py

@@ -206,4 +206,4 @@ def test_raise_if_missing_package_is_noop_when_present() -> None:
         raise_if_missing_package,
     )
 
-    raise_if_missing_package()  # must not raise
+    assert raise_if_missing_package() is None