jsonb_concat: make sure we always return a non-scalar value

There was a long thread about concatenating jsonb objects to each other,
but that discussion didn't touch concatenating other types. Currently
jsonb_concat always just returns the other argument if one of arguments
is considered empty. This causes surprising behavior when concatenating
scalar values to empty arrays:

os=# select '[]'::jsonb || '1'::jsonb;
1

os=# select '[]'::jsonb || '[1]'::jsonb;
[1]

os=# select '[]'::jsonb || '1'::jsonb || '2'::jsonb;
[1, 2]

os=# select '0'::jsonb || '1'::jsonb;
[0, 1]

os=# select '{"x": "y"}'::jsonb || '[1]'::jsonb;
[{"x": "y"}, 1]

os=# select '{"x": "y"}'::jsonb || '1'::jsonb;
ERROR: invalid concatenation of jsonb objects

Attached a patch to fix and test this. Also added a test case for
concatenating two scalar values which currently produces an array.. I'm
not sure that behavior makes sense, but didn't want to change that in
this patch as I guess someone could consider that feature useful.

/ Oskari