BUG(maps): fix accidental revert

The changes from 1292566 were accidentally reverted by 2aef719.

heracles/maps.py

@@ -399,9 +399,10 @@ def mapper(page: "CatalogPage") -> None:
 
             _map_real(wht, val, ipix, w, v)
 
-            ngal += page.size
-            wmean += (w - wmean).sum() / ngal
-            var += ((w * v) ** 2 - var).sum() / ngal
+            if page.size:
+                ngal += page.size
+                wmean += (w - wmean).sum() / ngal
+                var += ((w * v) ** 2 - var).sum() / ngal
 
         # the mapper function is yield-ed to be applied over the catalogue
         yield mapper
@@ -552,9 +553,10 @@ def mapper(page: "CatalogPage") -> None:
 
             _map_complex(wht, val, ipix, w, re, im)
 
-            ngal += page.size
-            wmean += (w - wmean).sum() / ngal
-            var += ((w * re) ** 2 + (w * im) ** 2 - var).sum() / ngal
+            if page.size:
+                ngal += page.size
+                wmean += (w - wmean).sum() / ngal
+                var += ((w * re) ** 2 + (w * im) ** 2 - var).sum() / ngal
 
         # the mapper function is yield-ed to be applied over the catalogue
         yield mapper