[ORKStepHeaderView] Fix nil attribute warning
[ORKStepHeaderView] Fix nil attribute warning

mluedke2/ResearchKit,chb/ResearchKit,jeremiahyan/ResearchKit,chb/ResearchKit,jeremiahyan/ResearchKit,jeremiahyan/ResearchKit,md0u80c9/ResearchKit,md0u80c9/ResearchKit,md0u80c9/ResearchKit,chb/ResearchKit,chb/ResearchKit,mluedke2/ResearchKit,md0u80c9/ResearchKit,mluedke2/ResearchKit