go.tools/present: correctly test empty argument
go.tools/present: correctly test empty argument

Updates issue 7613.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/105130046

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools