vinyl: fix crash when empty PK DDL races with DML

Vinyl doesn't support altering the primary index of a non-empty space,
but the check forbidding this isn't entirely reliable - the DDL function
may yield to wait for pending WAL writes to finish after ensuring that
the space doesn't contain any tuples. If a new tuples is inserted into
the space in the meantime, the DDL operation will proceed rebuilding
the primary index and trigger a crash because the code is written on
the assumption that it's rebuilding a secondary index:

```
./src/box/vinyl.c:1572: vy_check_is_unique_secondary_one: Assertion `lsm->index_id > 0' failed.
```

Let's fix this by moving the check after syncing on WAL.

Closes #10603

NO_DOC=bug fix

(cherry picked from commit 955537b57c2aade58b7ca42501a9bbe50dd91f26)