The melt merge_vcf step fails if all input files are empty.

[Nicolai-vKuegelgen]

Describe the bug
The merge_vcf wrapper script for melt output postprocessing takes several input vcf files, checks whcih are empty and merges the rest of them into the final output. However, if all of the input files are empty then the merging does not work properly and the script exits without proper output.

To Reproduce
Steps to reproduce the behavior:

1. Run snappy with sample for which melt output will be empty
2. See error from merge_vcf wrapper script

Expected behavior
The script should include and additional check if all input is empty so that properly formatted empty output files can be created in this case.

Additional context
N/A