fix(poly_simplify): set default ops to +

Author: justindujardin

libraries/mathy_python/mathy/envs/poly_simplify.py

@@ -21,6 +21,8 @@ class PolySimplify(MathyEnv):
 
     def __init__(self, ops: List[str] = None, **kwargs):
         super().__init__(**kwargs)
+        if ops is None:
+            ops = ["+"]
         self.ops = ops
 
     def get_env_namespace(self) -> str: