Add a filter for dropping variants without any calls

[arostamianfar]

We currently have the --omit_empty_sample_calls option, which drops empty calls. We can also add an option to drop the entire variant record if all calls are empty. This would be useful for large datasets where entire variant records are empty and are not needed for analysis.
Note that this should be done in the FilterVariants step as we want to avoid processing these variants as soon as possible (to save cost/time).