Allow null in SetDefaultValue (#914)

Author: KathleenDollard

src/System.CommandLine/Argument.cs

@@ -134,7 +134,7 @@ public Type ArgumentType
             return _defaultValueFactory.Invoke(argumentResult);
         }
 
-        public void SetDefaultValue(object value)
+        public void SetDefaultValue(object? value)
         {
             SetDefaultValueFactory(() => value);
         }

src/System.CommandLine/Option.cs

@@ -45,7 +45,7 @@ public virtual Argument Argument
         IArgument IOption.Argument => Argument;
 
         public bool Required { get; set; }
-
+ 
         string IValueDescriptor.ValueName => Name;
 
         Type IValueDescriptor.ValueType => Argument.ArgumentType;