Pass through None during param resolution (#5466)

- pass through None as a parameter if set.

Technically a breaking change, since this used to silently drop parameters that were set as None. 

Fixes: #4029

cirq-core/cirq/study/resolver.py

@@ -261,6 +261,8 @@ def _from_json_dict_(cls, param_dict, **kwargs):
 
 
 def _resolve_value(val: Any) -> Any:
+    if val is None:
+        return val
     if isinstance(val, numbers.Number) and not isinstance(val, sympy.Basic):
         return val
     if isinstance(val, sympy_numbers.IntegerConstant):

cirq-core/cirq/study/resolver_test.py

@@ -25,6 +25,7 @@
 @pytest.mark.parametrize(
     'val',
     [
+        None,
         3.2,
         np.float32(3.2),
         int(1),