fallback to default separators in number_format

When number_format receives 'null' for one of its separator parameters,
we must pass it back to t_number_format so it can decide whether or not
it should fallback to its default separators.

Currently, when passing NULL to a parameter that is defined as String,
it is implicitly converted to an empty string, and subsequently
str.isNull() will return false.

Instead, we use a Variant with a default string value.