Late and hard to follow failure for level2 if AOI does not overlap with inputs

[Hneuschmidt]

When queried and processed area of interest do not overlap, the process fails late (after processing all inputs) and with a strange failure message:

OpenEO batch job failed: JSONDecodeError('Expecting value: line 66 column 20 (char 1616)')

The reason is that the STAC templates assume that certain files are present (e.g CITEME) which do not appear if there is not output data.
STAC generation should gracefully handle incomplete datacubes.

Furthermore, we may consider validating inputs and failing early if there is no overlap between inputs and AOI.