DICOMParser: Fix crashes in DICOMAppHelper methods

Extend the pattern already seen in the PatientNameCallback and
StudyIDCallback methods to the StudyUIDCallback and GantryAngleCallback
methods of DICOMAppHelper. Avoid calling string char* constructor and
ReturnAsFloat method when val == nullptr. Use an empty string or a
float value of 0 instead.

(cherry picked from commit d29f2b50)