Database INSERT changes NULL value to empty string

Not a problem here. The issue is with foreign key id's that don't have to be entered. I'm finding that even if I set the default to NULL, a 0 is entered, so I decided to stick with 0.

Now that you say this, I think there's another side to it - unless I'm mistaken, you cannot have a foreign key reference to a NULL "value".

I'm quoting that word on purpose, because NULL is in fact not a value, but a token for lack of value and therefore a foreign key doesn't make sense - a reference can't point to nothing. This is not limited to databases, but there are a lot of other instances in DB programming where it becomes obvious, like COUNT(field_name) ignoring rows where field_name is NULL.