Fix hasShowcaseView() method in case there is no target set

Without this patch, the method will return true if the target is set to Target.NONE. In
this case showcaseX and showcaseY is set to -1 and hasNoTarget is set to true. Since
the method is used by the TextDrawer class, the text bounds were incorrect without
a target.
Merge pull request #177 from ejonny/master

Fix hasShowcaseView() method in case there is no target set