add specs for preventing nil zones in util presenters

refs CNVS-16799

No behavior change, just proving that when you
explicitly pass in a nil object to a date or time
presenter as a zone override, it uses the default
Time.zone correctly. the actual behavioaral change
went out in a hotfix in /c/44148

TEST PLAN: specs should pass

Change-Id: I59da9cf68d5738b1006ae1579db2946731468539
Reviewed-on: https://gerrit.instructure.com/44159
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>