txm: fix a bug with wrong space:count() result

Internally space's indexes can containt dirty tuples that are
invisible for user. That's why there's special adjustment in
space:count() that substracts number of invisible tuple in the
space.

By a mistake that check thought that all prepared statements are
visible, which is wrong for autocommit reads, like standalone
space:count() without explicit transaction.

Fix it by using common for all reads practice: ignore prepared
statements if current transaction is NULL.

Closes #6421
NO_DOC=bugfix