Do empty columns take up space in a table?
It takes minimal space when not used


one bit in the NULL bitmap
two bytes for length (which will be zero when NULL)


The overhead is minimal and optimisation will be premature.

Until you know you have an issue, just keep it in one table. You break KISS by introducing outer joins and add an overhead in querying the data.

See https://stackoverflow.com/questions/3793022/how-to-come-to-limits-of-8060-bytes-per-row-and-8000-per-varchar-nvarchar-valu/3793265#3793265 for more