Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• firebird_standard_use_type_backend::clean_up
• odbc_vector_into_type_backend::clean_up
• postgresql_standard_use_type_backend::clean_up

[mloskot]

It's harmless, but feel free to submit a pull request.

[elfring]

How do you think about to use smart pointers at affected source code places instead?

[mloskot]

Please, do not apply any C++ modernisations that would require C++11 or later.

If you wish to contribute, just remove the redundant if check, simple as that and please resist any large scale modernisations.