Updating Fiducial Parameters for Relative Binning when not using Calibration Model

This line which was a fix from !525 (merged) causes the data generation stage to fail when calibration-model=None. This is because this line will set self.calibration_prior to None, so the line linked above will try to iterate through None.

The simplest fix would probably be to only iterate through calibration-prior if it is not None.