There's a good conversation about it here: MySQL: NULL vs ""
I found out that NULL vs "" is insignificant in terms of disk-space and performance.
There's some others discussing that it's faster to search for a "null bit" than checking that a string is
''. That question is 9 years old, so we have come a long way hardware wise since then, and the difference is probably negligible. As a DBA, I'd recommend using nulls appropriately :)