text/template: revert minor change to Name method
For  better printing, I recently changed Name to return "<unnamed>" for templates
with empty names, but this causes trouble for the many packages that used "" as
the template name, so restore the old behavior.
It's usually printed as a quoted string anyway, so it should be fine.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/7577044