fix up legacy odb conversion

[bbrehm]

Another oops: We don't have unit tests in this repo for legacy odb conversion. Last PR broke that, and I only noticed in the codepropertygraph repo when bumping deps...

So this PR needs to go in to unbreak that. Afterwards: How do we want to deal with the unit test situation?

1. Write new tests here
2. copy/move over tests from the other repo
3. document that this is an issue, so that we know to run that test next time we touch serialization code

[mpollmeier]

Yes we should really have some tests here.
Given that odb is legacy and won't be change any more (as far as we can forsee now...), how about we add an odb database as a binary to src/resources, run odb2fg and verify some numbers.

There should also be some simple instructions on how we created that, which could be something like 'download joern version X and run ./joern-scan example-code and take the cpg.bin.