Stop bibtex from complaining about an empty note

As pointed out by @lockstep in the comments, note is one of the mandatory fields for the @unpublished entry type.

This warning will disappear if you change the entrytype from @unpublished to @generic, @conference, or @inproceedings.

Instead of changing the entry type you can also enter an actual note in the Extra field. For example, for the @Manuscript entry I usually enter 'working paper' or 'unpublished.