rgw: print dict-like IAM Policy element as a dict

rgw: print dict-like IAM Policy element as a dict

so they are printed like
{ Hello: World, Goodbye: [ Cruel, World ] }
instead of
[ Hello: World, Goodbye: [ Cruel, World ] ]

also, all of the print_dict() caller sites are protected by
`if (!foo.empty())`, so no need to check for empty.

Signed-off-by: Kefu Chai <kchai@redhat.com>