The `b64dec()` function decodes the given base64 input string.

Returns a string containing the decoded data.

Returns `null` if the input is not a string or if the input string was
invalid base64 data (e.g. missing padding or non-whitespace characters
outside the expected alphabet).

-- Testcase --
{%
	printf("%.J\n", [
		b64dec("SGVsbG8sIHdvcmxkIQ=="),
		b64dec("SGVsbG8sIHdvcmxkIQ"),
		b64dec("AAECAw=="),
		b64dec("xxx"),
		b64dec("==="),
		b64dec(true)
	]);
%}
-- End --

-- Expect stdout --
[
	"Hello, world!",
	null,
	"\u0000\u0001\u0002\u0003",
	null,
	null,
	null
]
-- End --