fix: LevelDB transaction nil panic

[nasdf]

Description

I forgot to add a nil check here. This PR also bumps the goleveldb version since it includes a fix for a data race.

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/validate-conventional-style.sh.
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Integration tests

Specify the platform(s) on which this was tested:

• MacOS

[fredcarle]

LGTM