BGP: "show ip bgp json" should return empty "routes : {}" if the table

BGP: "show ip bgp json" should return empty "routes : {}" if the table
is empty

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-7475
Reviewed By: Donald Sharp
Testing Done:

If the BGP table is empty, intead of returning

{'alert': 'No BGP network exists', 'routerId': '10.0.9.2',
'tableVersion': 180}

we should return

{'routerId': '10.0.9.2', 'routes': {}, 'tableVersion': 180}

This provides a more consistent json interface which is easier to script
against.