COMP: Suppress false positive non-null warning by GCC

The error message was:

In file included from /home/dzenan/ITKNornir/include/IRGridCommon.h:33,
                 from /home/dzenan/ITKNornir/src/IRGridCommon.cxx:30:
/home/dzenan/ITKNornir/include/itkIRCommon.h: In function ‘typename legendre_transform_t::Pointer approx_transform(const pnt2d_t&, const pnt2d_t&, const mask_t*, const base_transform_t*, unsigned int, unsigned int, bool) [with legendre_transform_t = itk::LegendrePolynomialTransform<double, 1>]’:
/home/dzenan/ITKNornir/include/itkIRCommon.h:6305:51: warning: ‘this’ pointer is null [-Wnonnull]
 6305 |   vec2d_t shift = center - inverse->TransformPoint(center);
      |                            ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

include/itkIRCommon.h

@@ -6302,7 +6302,10 @@ approx_transform(const pnt2d_t &          tile_min,
   // calculate the shift:
   pnt2d_t center =
     pnt2d(tile_min[0] + (tile_max[0] - tile_min[0]) / 2.0, tile_min[1] + (tile_max[1] - tile_min[1]) / 2.0);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnonnull"
   vec2d_t shift = center - inverse->TransformPoint(center);
+#pragma GCC diagnostic pop
 
   typename legendre_transform_t::Pointer approx = setup_transform<legendre_transform_t>(tile_min, tile_max);
   approx->setup_translation(shift[0], shift[1]);