Fix highlighting of results when the search result does not return termMatches

Fix highlighting of results when the search result does not return termMatches

If the search engine does not know the concept of serch terms the search
result object will just return an empty array as defined in class
SearchResultSet.

In this case SearchHighlighter::highlightSimple will place a span between
each and every byte (yes, byte, not character, it will break multibyte
chars).

This patch will just output the first few lines of a page if no search
terms are available for highlighting.

Note: Highlighting the page name in the case of title matches is
questionable, IMHO. It might make more sense for this case as well to
just return the first few lines of the page.

Change-Id: I276418f271855fb99443188f51cc076289c6ba0d