Default checksum algorithms not triggered

[gabrielefronze]

Motivation

If the custom checksum support RSE attribute supported_checksums is not set, the RSE is by default defined compatible with the standard checksum algorithms.

Due to how get_rse_attribute works, when supported_checksums is not found, an empty string containing is returned instead of a None string.

The following logic assumed to have an None string, instead of a string formatted like that.

Modification

Update the logic to sense such behavior and return the standard GLOBALLY_SUPPORTED_CHECKSUMS list in that case.