compiler: remove unneeded zero-length slice value init
compiler: remove unneeded zero-length slice value init

Prune away a bit of unnecessary code from the helper routine
Slice_construction_expression::create_array_val() that was adding
an extra NULL value to empty slices (no longer needed).

Change-Id: I830645f91db65248c4a2d37d7d0dd3101484a780
Reviewed-on: https://go-review.googlesource.com/34410
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend