sql: soft return if space isn't found by id

Recently, spaces for statistics have become system. Upgrade between
schemas occurs after restoring from snapshot and initializing SQL
system.  While restoring from old snapshot, there are not yet stat
tables in Tarantool, but they exists in SQL as internal representation
of system tables. Thus, SQL system initializing attempts at fetching
values from stat spaces into internal structures, meanwhile there are no
stat spaces in Tarantool yet, so look up by space id leads to fail and
dereference of NULL pointer.  This patch makes checks correctness of
space after look up and returns if it isn't found.

Closes #3153