StringConverter returns value other than string

[araj-dev]

GrimoireJS/src/Converters/StringConverter.ts

Lines 4 to 6 in 9298762

	} else if (!val) {
	return val;
	} else if (typeof val.toString === "function") {

If a value such as 0 or false is passed to this function, it can return value other than string.