Incorrect serialization for "Command to rerun" for BoolParameter and OptionalParameter

[arteymix]

BoolParameter appear as "False" in the GUI. They should be omitted if False and present otherwise without a value. There is also the parsing attribute to take into consideration, as an explicit bool parameter expects true or false (and not the title-case one).

OptionalParameter appear as "". I think they should be simply omitted when the value of the task is None.

As an example, in our pipeline, the --rerun option is a boolean and --srr is optional.

Running this command as-is produces an error.

$ luigi --module rnaseq_pipeline.sources.sra rnaseq_pipeline.sources.sra.DownloadSraExperiment --metadata "{\"experiment_id\": \"GSE266876\", \"sample_id\": \"GSM8255390\"}" --rerun "False" --srx "SRX24426707" --srr ""
usage: luigi [--local-scheduler] [--module CORE_MODULE] [--help] [--help-all] [--rnaseq-pipeline.sources.sra.DownloadSraExperiment-metadata RNASEQ_PIPELINE.SOURCES.SRA.DOWNLOADSRAEXPERIMENT_METADATA]
             [--metadata METADATA] [--rnaseq-pipeline.sources.sra.DownloadSraExperiment-rerun] [--rerun]
             [--rnaseq-pipeline.sources.sra.DownloadSraExperiment-srx RNASEQ_PIPELINE.SOURCES.SRA.DOWNLOADSRAEXPERIMENT_SRX] [--srx SRX]
             [--rnaseq-pipeline.sources.sra.DownloadSraExperiment-srr RNASEQ_PIPELINE.SOURCES.SRA.DOWNLOADSRAEXPERIMENT_SRR] [--srr SRR]
             [Required root task]
luigi: error: unrecognized arguments: False