FIX/MNT: Simplify project geometry handling

The MultiLineString return type was a plain list, but it should be
an empty MultiLineString to be consistent with the other types.
Additionally, all geometry constructors take empty lists, so
just use that rather than special-casing the returns.

lib/cartopy/crs.py

@@ -918,33 +918,23 @@ def _project_multipoint(self, geometry, src_crs):
         geoms = []
         for geom in geometry.geoms:
             geoms.append(self._project_point(geom, src_crs))
-        if geoms:
-            return sgeom.MultiPoint(geoms)
-        else:
-            return sgeom.MultiPoint()
+        return sgeom.MultiPoint(geoms)
 
     def _project_multiline(self, geometry, src_crs):
         geoms = []
         for geom in geometry.geoms:
             r = self._project_line_string(geom, src_crs)
             if r:
                 geoms.extend(r.geoms)
-        if geoms:
-            return sgeom.MultiLineString(geoms)
-        else:
-            return []
+        return sgeom.MultiLineString(geoms)
 
     def _project_multipolygon(self, geometry, src_crs):
         geoms = []
         for geom in geometry.geoms:
             r = self._project_polygon(geom, src_crs)
             if r:
                 geoms.extend(r.geoms)
-        if geoms:
-            result = sgeom.MultiPolygon(geoms)
-        else:
-            result = sgeom.MultiPolygon()
-        return result
+        return sgeom.MultiPolygon(geoms)
 
     def _project_polygon(self, polygon, src_crs):
         """

lib/cartopy/tests/test_line_string.py

@@ -73,6 +73,13 @@ def test_out_of_domain_efficiency(self):
         tgt_proj.project_geometry(line_string, src_proj)
         assert time.time() < cutoff_time, 'Projection took too long'
 
+    def test_multi_linestring_return_type(self):
+        # Check that the return type of project_geometry is a MultiLineString
+        # and not an empty list
+        multi_line_string = ccrs.Mercator().project_geometry(
+            sgeom.MultiLineString(), ccrs.PlateCarree())
+        assert isinstance(multi_line_string, sgeom.MultiLineString)
+
 
 class FakeProjection(ccrs.PlateCarree):
     def __init__(self, left_offset=0, right_offset=0):