Export Attendance file corrupted if Manager's email not set

Steps to reproduce:
1. Set facetoface_addchangemanageremail global setting to "No" in site admin > modules > activities > face-to-face
2. Create a face-to-face (uncheck all send notice to manager boxes)
3. Export attendance (Excel or OpenDocument format)

What should happen:
File contains face-to-face attendance details

What actually happens:
File corrupted. See attached file for example.

Fix:
Bug is caused by attempt to write an object property that has not been set. Attached patch (download-attendance.patch.txt) provides fix by checking property is set before writing.