Fixed bug in code to ensure pipeline continued even if nothing to impute

If impute 2 got an interval with nothing to impute then it would not
create a file for that interval, which snakemake required. Had tried
to correct this with "impute2-command || touch output_file", but this failed
since impute2 exits cleanly even if there are no snps to impute so had
to use "impute2-command; touch output_file" instead.