always add `dssp_version` argument

[edikedik]

Hi,

I had a nasty bug, such that the DSSP features are silently omitted. This happens due to biopython returning an empty dictionary for newer DSSP versions, for which the old "dssp" format must be specified explicitly. As a result, the dssp_df becomes empty, and the required node features are not added, without throwing any errors (btw, at least checking that the "dssp_df" is not empty might be a good practice).

Anyway, taking this line (https://github.com/a-r-j/graphein/blob/27463a5ca0b9714ba131486e0838eceebcb1ea15/graphein/protein/features/nodes/dssp.py#L113) out of the if-else expression solves this issue.