Avoid creating references to uninitialized data

[Lorak-mmk]

Creating a reference to uninitialized data is most of the time undefined behaviour in Rust - even if data under this reference is never read.

This commit removes all occurences of this problem (in accordance with cpp-driver's semantics), instead using std::ptr::write - safer and more explicit way to achieve the same goal.

Pre-review checklist

• [x] I have split my patch into logically separate commits.
• [x] All commit messages clearly explain what they change and why.
• [x] PR description sums up the changes and reasons why they should be introduced.
• [ ] I have enabled appropriate tests in .github/workflows/build.yml in gtest_filter.
• [ ] I have enabled appropriate tests in .github/workflows/cassandra.yml in gtest_filter.

[Lorak-mmk]

it is not obvious from the documentation what will happen if it gets a null pointer as a destination location

Documentation states it is undefined behavior.

cppdriver also doesn't check for null outputs and would also exhibit UB in that case, so there is no regression on that front. I can add null checks, but it would be just a hardening measure, not required for corectness of this project.