Fix sumset docstring (#15038)

* Fix sumset docstring

* Add empty set test for sumset

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Co-authored-by: Christian Clauss <cclauss@me.com>

Author: sanchiagarwal0

maths/sumset.py

@@ -24,6 +24,8 @@ def sumset(set_a: set, set_b: set) -> set:
     Traceback (most recent call last):
     ...
     AssertionError: The input value of [set_b=3] is not a set
+    >>> sumset(set(), {1, 2, 3})
+    set()
     """
     assert isinstance(set_a, set), f"The input value of [set_a={set_a}] is not a set"
     assert isinstance(set_b, set), f"The input value of [set_b={set_b}] is not a set"