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.

(cherry picked from commit 076d9b2a)