Fix $compile text node issue in IE7/8

IE7/8 inconsistency adds and removes empty text nodes between block,
inline, and comment elements.  These implicit nodes may appear or vanish
after directive compilation, which breaks the correlation between the
nodeList and nodeLinkFn arrays within compositeLinkFn.

This commit simply ignores empty text modes both during compilation and
linking under the assumption that an empty text node will never have an
associated directive.