[DDC-2310] Fix evaluation of NOLOCK table hint on SQL Server

This PR is complementary to https://github.com/doctrine/doctrine2/pull/910.
ORM passes `null` to `AbstractPlatform::appendLockHint()` as `$lockMode` which should not evaluated to `LockMode::NONE` unless `0` is explictly given. Otherwise ORM appends `WITH (NOLOCK)` to all queries even though, no query lock hint is set.