revert: fix(ngHref): remove attribute when empty value instead of ignoring

This reverts commit 948c86c6025fca8e07921869d21cfac1c6333b05.

This commit caused tests to fail in IE8 due to a TypeError '0.childNodes is
null or not an object. The issue should be investigated and fixed. Issue #8340
has been opened to investigate.

See this job for failures: https://travis-ci.org/angular/angular.js/jobs/30792508