go command refuses to use GOPATH with empty path elements
(like %GOPATH%=C:\go;). But environment variable change dialog
on Windows 10 produces strings ending with ; (see issue #21928
for a picture). Just accept GOPATH with empty path elements,
and ignore all empty path elements.

Fixes #21928

Change-Id: I1d3c3a19274ed69204d29ae06c3e8ff8c57c1ca0
Reviewed-on: https://go-review.googlesource.com/65151
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>