CSSStyleRule.style.setProperty() sets stringified 'undefined' / 'null'

[asamuzaK]

CSSStyleRule.style.setProperty('color', undefined) sets stringified 'undefined' as it's value.
Also, CSSStyleRule.style.setProperty('color', null) sets stringified 'null' as it's value.

Related #189

[asamuzaK]

Test summary:

STR	Expected	Actual
cssRule.style.setProperty("background-color", "green"); cssRule.style.setProperty("background-color", undefined);	"green"	"undefined"
div.style.setProperty("background-color", "green"); div.style.setProperty("background-color", undefined);	"green"	"green"
div.style.backgroundColor = "green"; div.style.backgroundColor = undefined;	"green"	TypeError
div.style.setProperty("--custom", "green"); div.style.setProperty("--custom", undefined);	"undefined"	""
cssRule.style.setProperty("background-color", "purple"); cssRule.style.setProperty("background-color", null);	""	"null"
div.style.setProperty("background-color", "purple"); div.style.setProperty("background-color", null);	""	""
div.style.backgroundColor = "purple"; div.style.backgroundColor = null;	""	""
div.style.setProperty("--custom", { toString: () => null });	"null"	null