Tolerate null values passed to Kti::create()

[Philipp91]

The fields are actually nullable and in ::fromAccount() we might receive nulls.

Fixes #405

[timrasche]

Please merge!

[bastidest]

I have hit the same issue with the downstream project https://github.com/bnw/firefly-iii-fints-importer . I tried this patch and it fixes the issue.