Automattic/jetpack — pull request #1294 by dd32 (2014-11-13 22:28:43+00:00)
Title: Notes Module: Avoid a PHP Notice when the request doesn't contain a User-Agent header

Not all requests include a User-Agent, although all requests which Notes are displayed on should.
This small change simply checks that a user-agent exists before the IE checks are run, avoiding the notice all together.