res_realtime: Fix 'realtime update2' argument handling

res_realtime: Fix 'realtime update2' argument handling

The change in 9b99ef50b5d01ee8111d26efa7b926bdfaf3f980 updated the
syntax of the 'realtime update2' CLI command but did not correctly
update the calls to ast_update2_realtime().

The issue this addresses was originally opened because we aren't
allowing a SQL NULL to be set as part of the update, but this is a
limitation of the Realtime API and is not a bug.

Additionally, this patch:

* Corrects the example in the command documentation to reflect
  'update2' instead of 'update.'

* Fixes the leading spacing of the command documentation.

* Checks that the required 'NULL' literal argument is present where we
  expect it to be.

ASTERISK-21794 #close
Reported by: Cédric Bassaget

Change-Id: Idda63a5dc50d5f9bcb34c27ea3238d90f733b2cd