Remove Nullable disable from Png Code

[stefannikolei]

I added a new Property to PngChunk --> DataSpan. On all places Data was used the same way. So we have the guard on this place and can just use it.

Prerequisites

• [ ] I have written a descriptive pull-request title
• [ ] I have verified that there are no overlapping pull-requests open
• [ ] I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [ ] I have provided test coverage for my change (where applicable)

Description