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.

[vgonkivs]

There it is — line 243: n.addLeaf(j.sharePos, nil). When GetNode fails, it adds a nil leaf to the namespace data. The error is stored via singleErr.Do but processing continues — other leaves may succeed.
Then at line 225, retrievalErr is returned but Leaves() still contains nil entries from failed retrievals. So the nil check in GetSharesByNamespace is not redundant — it protects against panicking on these nil leaves.