go/parser: always provide a non-nil path for imports
go/parser: always provide a non-nil path for imports

The go/ast ImportSpec always requires a non-nil path.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/10402047

gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme