The `join()` function constructs a string out of the given array by
converting each array item into a string and then joining these substrings
putting the given separator value in between. An empty array will result in
an empty string.

The separator argument is converted into a string in case it is not already
a string value.

Returns `null` if the given array argument is not an array value.

-- Testcase --
{%
	printf("%.J\n", [
		join("|", []),
		join("|", [ 1, 2, 3 ]),
		join("|", [ null, false, "" ]),
		join(123, [ "a", "b", "c" ]),
		join(123, { "not": "an", "array": "value" })
	]);
%}
-- End --

-- Expect stdout --
[
	"",
	"1|2|3",
	"null|false|",
	"a123b123c",
	null
]
-- End --