Removed requirement for note.data to be present

https://dev.metasploit.com/redmine/issues/5840
https://www.pivotaltracker.com/story/show/53702805

### Steps to verify
- [x] Check out MSF branch, Pro should be on develop and up to date
- [x] Have some workspace in your Pro DB with hosts and notes (add a note on a host if you lack one)
- [x] cd pro/engine && ./msfpro
- [x] Be in the workspace that has hosts and notes, run `db_export /tmp/foo`
- [x] Once complete, edit the file, find a <note> element; within it, find a <data> element and remove any text between the start and end tags. I.e. you should end up with <note>...<data></data>...</note>
- [x] Back in msfpro, import your edited file with `db_import /tmp/foo`
- [x] There should be no errors, import should complete