fix: move non-NaN enforcement inside contain_nans block

Author: BitForge95

tests/create_sequence_data.py

@@ -49,10 +49,10 @@ def _generate_sequence_data(
             data.size, size=int(0.1 * data.size), replace=False
         )
         data.flat[nan_indices] = np.nan
-
-    # ensure each row has at least one non-NaN
-    row_all_nan = np.isnan(data).all(axis=1)
-    data[row_all_nan, 0] = 0.0
+        # ensure each row has at least one non-NaN
+        if n_signals > 0:
+            row_all_nan = np.isnan(data).all(axis=1)
+            data[row_all_nan, 0] = 0.0
 
     if not use_mem_mapped:
         np.save(sequence_root / "data.npy", data)