initialize return value for deterministic queries

[bashbaug]

This is a change I have been using locally for some time now that I've found useful, so I thought I'd see if it's something worth adding to the upstream C++ bindings. In short, for queries that return the result of the query directly (vs. through a pointer), the value that is returned is uninitialized. This means that if the query returns an error, the value that gets returned is most likely random and non-deterministic, which likely isn't what users expect.

With this change, the return values are always initialized, so even if an error occurs the query always returns a deterministic value. The return value is not always meaningful, or identifiable as a default value, but it frequently is, and even when it isn't it's beneficial to return a consistent value than a random value.

[kpet]

Merging as discussed in 2025/09/19 teleconference.