fix failing hr_attendance_analysis tests

the tests would fail when run after the 11th of the month
because then the hr.contract created by the demo data would not be valid at the
time of the timesheet.

* change the start date of the contract in the tests
* fix HrAttendance.get_default_calendar to raise an exception when no contract
  is found instead of returning an empty list which caused an AttributeError
fix long line
don't raise exception when no contract found

this would cause a failure at addon installation time.
We keep the initial logic (no error) but return a browse null
instead of an empty list.
Merge pull request #10 from gurneyalex/7.0-fix_hr_attendance_analysis

[FIX] hr_attendance_analysis: Fix tests