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. From the users point of
view I expect the empty string to behave like null/undefined.

This is a resubmit of I331924d without the problematic bits.

Change-Id: I07847600bb24ae078276acf98e6eb244039414d7