Do not allow creating/importing investigation with no title

[mikkonie]

This is the root cause of e.g. #1836. I don't think there is a valid reason to have an empty title.

[mikkonie]

Turns out #1836 is fixable without doing this, so we can postpone and decide on this later. As Thomas pointed out, the investigation title is not explicitly required by the ISA standard. The question still remains if there is a viable case for not having one.