compiler: use nil pointer for zero length string constant
compiler: use nil pointer for zero length string constant

We used to pointlessly set the pointer of a zero length string
constant to point to a zero byte constant.  Instead, just use nil.

Change-Id: I73fb28dcfcf547565dcab4ec26412f6e6e7ca951
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/384354
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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