app: restore behavior of gimp_gegl_apply_operation() with NULL src_buffer

Restore the behavior of gimp_gegl_apply_operation() prior to
11629fde 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.
5 jobs for master in 24 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Dependencies
passed #456267
deps-git

00:02:27

passed #456268
deps-x86_64-w64-mingw32

00:13:24

 
  Gimp
passed #456269
gimp-autotools

00:10:37

passed #456270
gimp-meson

00:07:11

passed #456271
gimp-x86_64-w64-mingw32-meson

00:09:31