fix(ngSrc): don't set src if value is empty string

Current implementation of ngSrc may lead to empty src attribute when page is loading.

For example:

<img ng-src="{{image.url}}">
can be temporarily rendered as

<img src="">
before the image resource is loaded.

Some browser emits a request to the current page when seeing <img src=""> (Firefox13 and IE8 will, Chromium20 won't), which leads to performance problems.