Fixed GIS test failures when GDAL is not installed

Thanks Iacopo Spalletti for the report and testing.

django/contrib/gis/geos/geometry.py

@@ -493,6 +493,9 @@ class GEOSGeometry(GEOSBase, ListMixin):
             else:
                 return
 
+        if not gdal.HAS_GDAL:
+            raise GEOSException("GDAL library is not available to transform() geometry.")
+
         if isinstance(ct, gdal.CoordTransform):
             # We don't care about SRID because CoordTransform presupposes
             # source SRS.
@@ -500,9 +503,6 @@ class GEOSGeometry(GEOSBase, ListMixin):
         elif srid is None or srid < 0:
             raise GEOSException("Calling transform() with no SRID set is not supported")
 
-        if not gdal.HAS_GDAL:
-            raise GEOSException("GDAL library is not available to transform() geometry.")
-
         # Creating an OGR Geometry, which is then transformed.
         g = gdal.OGRGeometry(self.wkb, srid)
         g.transform(ct)

tests/gis_tests/geos_tests/test_geos.py

@@ -989,6 +989,7 @@ class GEOSTest(unittest.TestCase, TestDataMixin):
             self.assertEqual(g1.srid, 4326)
             self.assertIsNot(g1, g, "Clone didn't happen")
 
+    @skipUnless(HAS_GDAL, "GDAL is required.")
     def test_transform_nosrid(self):
         """ Testing `transform` method (no SRID or negative SRID) """