Fix returning no mask when XTDF slicing is enabled but entire train selected

src/exdf/write/sd_writer.py

@@ -540,7 +540,7 @@ def mask_index(g, counts, masks_by_train):
         g['first'][:] = get_pulse_offsets(counts)
         g['count'][:] = counts
 
-        return full_mask
+    return full_mask
 
 
 def read_keydata(kd):