fix: remove redundant nil check and fix comment

[Bashmunta]

The nil-check for leaves in GetSharesByNamespace was unnecessary because CollectLeavesByNamespace returns an error on any retrieval failure, and the absent-namespace path exits before building shares. This could silently skip elements if retained. Also corrected the function comment to reflect actual behavior: errors short-circuit with no shares, while absent namespace returns no shares and a proof with err == nil.