Bug in comparison of empty jsonb arrays to scalars

Hi hackers.

While working on jsonbstatistics, I found the following bug:
an empty jsonb array is considered to be lesser than any scalar,
but it is expected that objects > arrays > scalars.

# select '[]'::jsonb < 'null'::jsonb;
?column?
----------
t
(1 row)

Attached patch contains:
1. bug fix (added the missing "else" in compareJsonbContainers())
2. regression test
3. pg_upgrade: invalidation of btree indexes on jsonb columns and
REINDEX-script generation

--
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company