[vbezhenar]

With empty string there's gotcha: Oracle treats empty string and null as the same values. If you really need sentinel value, generate UUID.

[mst]

I believe that's a nasty implementation detail of Oracle and not a problem for anybody using anything else.

A generated UUID, if possible, is probably much more clear because it will only be there if it was inserted very deliberately.

But at that point honestly I'd usually prefer breaking the field out into its own table.