[#4323] Fix project/<id>/reports/period-dates/
[#4323] Fix project/<id>/reports/period-dates/

Some periods may have no start or end dates, and this causes the
period-dates end-point to give a 500. This causes the reports page to
not load.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr