[gimp] app: restore behavior of gimp_gegl_apply_operation() with NULL src_buffer

[Ell <ell src gnome org>]

commit 076d9b2a283eb52b7f2fa10c4e92f12467ba1782
Author: Ell <ell_se yahoo com>
Date:   Sat Oct 5 09:37:00 2019 +0300

    app: restore behavior of gimp_gegl_apply_operation() with NULL src_buffer
    
    Restore the behavior of gimp_gegl_apply_operation() prior to
    11629fde660a97c695c04801f8997caedfdcc389 when src_buffer is NULL:
    keep the existing operation-node input, instead of using an empty
    input.  Unlike gimp_gegl_apply_cached_operation(),
    gimp_gegl_apply_operation() doesn't have an explicit
    connect_src_buffer parameter.
    
    This fixes empty output when merging layers.

 app/gegl/gimp-gegl-apply-operation.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/app/gegl/gimp-gegl-apply-operation.c b/app/gegl/gimp-gegl-apply-operation.c
index 43b0b053cb..13be25fe24 100644
--- a/app/gegl/gimp-gegl-apply-operation.c
+++ b/app/gegl/gimp-gegl-apply-operation.c
@@ -62,7 +62,7 @@ gimp_gegl_apply_operation (GeglBuffer          *src_buffer,
   gimp_gegl_apply_cached_operation (src_buffer,
                                     progress, undo_desc,
                                     operation,
-                                    TRUE,
+                                    src_buffer != NULL,
                                     dest_buffer,
                                     dest_rect,
                                     crop_input,