Ensure that we skip empty maps that should not be appended.

This also reorders the initialization operation to ensure that
tract maps are built prior to any filtering of bad inputs.

python/lsst/pipe/tasks/healSparseMapping.py

@@ -546,9 +546,29 @@ def run(self, sky_map, tract, band, coadd_dict, input_map_dict, visit_summary_di
             patch_info = tract_info[patch]
 
             input_map = input_map_dict[patch].get()
+
+            # Initialize the tract maps as soon as we have the first input
+            # map for getting nside information.
+            if not tract_maps_initialized:
+                # We use the first input map nside information to initialize
+                # the tract maps
+                nside_coverage = self._compute_nside_coverage_tract(tract_info)
+                nside = input_map.nside_sparse
+
+                do_compute_approx_psf = False
+                # Initialize the tract maps
+                for property_map in self.property_maps:
+                    property_map.initialize_tract_maps(nside_coverage, nside)
+                    if property_map.requires_psf:
+                        do_compute_approx_psf = True
+
+                tract_maps_initialized = True
+
             if input_map.valid_pixels.size == 0:
                 self.log.warning("No valid pixels for band %s, tract %d, patch %d; skipping.",
                                  band, tract, patch)
+                continue
+
             coadd_photo_calib = coadd_dict[patch].get(component="photoCalib")
             coadd_inputs = coadd_dict[patch].get(component="coaddInputs")
 
@@ -568,21 +588,6 @@ def run(self, sky_map, tract, band, coadd_dict, input_map_dict, visit_summary_di
                 patch_poly_map = patch_poly.get_map_like(input_map)
                 input_map = hsp.and_intersection([input_map, patch_poly_map])
 
-            if not tract_maps_initialized:
-                # We use the first input map nside information to initialize
-                # the tract maps
-                nside_coverage = self._compute_nside_coverage_tract(tract_info)
-                nside = input_map.nside_sparse
-
-                do_compute_approx_psf = False
-                # Initialize the tract maps
-                for property_map in self.property_maps:
-                    property_map.initialize_tract_maps(nside_coverage, nside)
-                    if property_map.requires_psf:
-                        do_compute_approx_psf = True
-
-                tract_maps_initialized = True
-
             valid_pixels, vpix_ra, vpix_dec = input_map.valid_pixels_pos(return_pixels=True)
 
             # Check if there are no valid pixels for the inner (unique) patch region