Don't crash on an empty vcf

[kdm9]

Hi folks,

In samplot vcf, the accumulator variable for the number of parsed variants uses enumerate(), which means that the var_count variable wasn't define if there were no variants in the file. Likely, an empty vcf should be an error that is reported somewhere, but for now I at least define the variable so as not to get a rather inscrutable NameError.

Thanks K