vinyl: fix invalid tuple in surrogate delete

If the map contains non-string keys that cannot be used in the
JSON index, an incorrect tuple is created in
vy_stmt_new_surrogate_delete_raw during the construction of the
surrogate tuple. This occurs because the number of items that
the map contains has already been copied, while such invalid
key-value pairs are not copied.
The problem is resolved through the creation of dummy nil:nil
pairs in such cases.