Scan fails to load when SpacingBetweenSlices is undefined

[strixy16]

Lines 72 to 74

slice_spacing = float(self.metadata.get("SpacingBetweenSlices") or 0)
    if (not slice_spacing) or (slice_spacing >= 0):
        return

In the LUAD-CT-Survival dataset, some of the CT scans do not have the SpacingBetweenSlices defined

(0018,0088) DS SpacingBetweenSlices = <undefined>

When read in by SimpleITK, it gets stored as a string "None" in the metadata. So the code above fails because it cannot convert this into a float.

Example samples from LUAD-CT-Survival include:

• QIN-LSC-0014
• R0004
• R0013