JBIDE-17849 CSS Hyperlinks aren't working on non-WTP projects

An utility helper method is added due to find pages in projects that are not the WTP ones
(has no web-roots that can be achieved by WTP methods)

Signed-off-by: vrubezhny <vrubezhny@exadel.com>
Throw IllegalArgumentException if the context of filePath is null instead.