ImageDataset: fix handling of missing values in the case of an HDF5 dataset

it close #144 (closed)

The bug can be reproduct with the following file: scan_to_concateante_darfix_concat.h5

• Warning 1: when running from main it will fall into a first bug fixed by 271d0e18 Then only on the bug described in #144 (closed)
• Warning 2: With the provided dataset (and bug fix) the dimension fitting cannot be applied. This is expected because motor position are some random values without meaning.

Maybe it should target branch 1.1