Make an empty "?action=" parameter default to "view"

Make an empty "?action=" parameter default to "view"

When no action is given, e.g. in
https://en.wikipedia.org/wiki/URL
the action defaults to "view". Just like you called
https://en.wikipedia.org/wiki/URL?action=view

But when the action is empty, e.g.
https://en.wikipedia.org/wiki/URL?action=
you get an error message telling you that "the action specified"
can not be "recognized". Wait. I did not "specified" an action.
That's why I left the parameter empty. It doesn't make much sense
to have an empty action name that's different from the "view"
default, right? From the users point of view I expect the empty
string to behave like null/undefined.

Change-Id: I331924d9223e597293bc8285f0c330edf08e429b