Fix crash when exporting to SEG-Y with missing axis=0 chunk in selection_mask

[tasansal]

This PR addresses a crash that occurred when exporting to SEG-Y format and an entire axis=0 chunk was missing from the selection_mask. The following changes have been made in the mdio_to_segy function within mdio.py:

1. Added a condition to handle missing whole blocks by filtering them out from the flat_files array.
2. Concatenate the output SEG-Y file with the filtered flat_files list.