Allow empty analyses in import

Background

Currently, VEP fails if there are no called variants in a sample/region (either no actual variants or insufficient coverage), leaving ELLA users with a failed import (e.g. custom reanalysis with single gene) and a cryptic error message (https://github.com/Ensembl/ensembl-vep/issues/215)

Implementation

A possible fix is described in https://github.com/googlegenomics/gcp-variant-transforms/pull/502 (telling VEP of the format, rather than having VEP figuring it from file content, which it can't since there's no data, only header info. Imports with no variants should somehow be completed, so the user will have access to the analysis with bam files and technical info. This should be restricted to this particular use case, i.e. no variants and not due to other import errors.