Make Impact.impact_at_reg support all zero impacts (#773)

* Fix a bug where impact_at_reg would not work for all zero impacts
* Update CHANGELOG.md

Author: peanutfun

CHANGELOG.md

@@ -64,6 +64,7 @@ Removed:
 - Problem with `pyproj.CRS` as `Impact` attribute, [#706](https://github.com/CLIMADA-project/climada_python/issues/706). Now CRS is always stored as `str` in WKT format.
 - Correctly handle assertion errors in `Centroids.values_from_vector_files` and fix the associated test [#768](https://github.com/CLIMADA-project/climada_python/pull/768/)
 - Text in `Forecast` class plots can now be adjusted [#769](https://github.com/CLIMADA-project/climada_python/issues/769)
+- `Impact.impact_at_reg` now supports impact matrices where all entries are zero [#773](https://github.com/CLIMADA-project/climada_python/pull/773)
 
 ### Deprecated
 

climada/engine/impact.py

@@ -434,7 +434,7 @@ def impact_at_reg(self, agg_regions=None):
             Contains the aggregated data per event.
             Rows: Hazard events. Columns: Aggregation regions.
         """
-        if self.imp_mat.nnz == 0:
+        if np.prod(self.imp_mat.shape) == 0:
             raise ValueError(
                 "The aggregated impact cannot be computed as no Impact.imp_mat was "
                 "stored during the impact calculation"

climada/engine/test/test_impact.py

@@ -562,9 +562,13 @@ def test_admin0(self):
 
     def test_no_imp_mat(self):
         """Check error if no impact matrix is stored"""
-        # Test error when no imp_mat is stored
-        self.imp.imp_mat = sparse.csr_matrix((0, 0))
+        # A matrix with only zeros should work!
+        self.imp.imp_mat = sparse.csr_matrix(np.zeros_like(self.imp.imp_mat.toarray()))
+        at_reg = self.imp.impact_at_reg(["A", "A"])
+        self.assertEqual(at_reg["A"].sum(), 0)
 
+        # An empty matrix should not work
+        self.imp.imp_mat = sparse.csr_matrix((0, 0))
         with self.assertRaises(ValueError) as cm:
             self.imp.impact_at_reg()
         self.assertIn("no Impact.imp_mat was stored", str(cm.exception))