Recognize when harvesters do not record any datasets and fix the energieatlas-st harvester

@OC000021809054 Turns out the discrepancy between 88 harvests and 87 sources was indeed a bug: energieatlas-st did not record any datasets and hence was missing in the sources metric. This MR generally errs out in that case and does a minimal fix to the harvester so that it yields some results. I am not sure, but I suspect that this one will need proper rewriting when it is ported to Rust, but not now.