JabRef version 3.6dev
Steps to reproduce:

1. Create an entry with no BibTeyKey but otherwise all fields set
2. run Integrity Check
3. Integrity check reports no problem

Warning about empty BibtexKey occurs only at startup. However, the integrity checker should report the empty bibtex key, too.

@Patent{,
  author    = {Abraham Lincoln},
  title     = {The Vampire Hunter},
  number    = {12345677},
  year      = {1978},
}