Don't compare hashes of empty files in the diff command

Empty files may or may not have a hash depending if the -hash option is used
during backup.

src/duplicacy_snapshotmanager.go

@@ -1481,9 +1481,13 @@ func (manager *SnapshotManager) Diff(top string, snapshotID string, revisions []
 					} else {
 						same = right.IsSameAs(left)
 					}
+				} else {
+					if left.Size == 0 && right.Size == 0 {
+						same = true
 					} else {
 						same = left.Hash == right.Hash
 					}
+				}
 
 				if !same {
 					LOG_INFO("SNAPSHOT_DIFF", "  %s", left.String(maxSizeDigits))