when appending a text node to a <table> element, jquery crashes.

This is the sister pull request to https://github.com/jquery/jquery/pull/1442

When appending a text node to a `<table>`, jquery crashes during the call to manipulationTarget(). This change fixes the issue and averts the crash.

manipulationTarget() is trying to determine if the content being appended to the table is a `<tr>` tag. When doing this, it checks the nodeName of the appended content. Since the textnode doesn't contain any children, the nodeName() call crashes.

This commit ensures a firstChild property exists before sending it to nodeName().