Fix handling of vaex not being present

Author: marthacryan

tests/test_optional/test_px/test_px_input.py

@@ -14,7 +14,7 @@
 import warnings
 
 
-if optional_imports.get_module("vaex"):
+if optional_imports.get_module("vaex") is None:
     TEST_LIBS = ["polars"]
 else:
     TEST_LIBS = ["vaex", "polars"]