wpe: Use `about:blank` as default URL to support only using load-bytes
WebKit is not going to render anything until a URI is set, leading to a
WPE posting a WPE View did not render a buffer
error message. To avoid
requiring the user to know it if they only want to use
wpesrc::load-bytes
we can just use about:blank
as default and
everything will work as users would expect.