update twas filtering logic and revise null issue

[rfeng2023]

Two changes in this PR:

1. extract_regional_data function: To my understanding, once added --region-name, we should focus on this region only as 'filtering' instead of adding additional region by pd.contact
2. revised NULL issue with a safe check and filter

[rfeng2023]

@Chunmingl @gaow please review this PR and let me know if you find any issue, thank you!

[Chunmingl]

Hi, @rfeng2023
Thanks for catching them.
The reason of the pd.contact was to allow --region-namehandle multiple region names for flexible job submission, but the functionality of it probably is not essential. I am ok with remove and simplify it.

you're right about the xqtl_meta_df[!duplicated(xqtl_meta_df[, c(\"region_id\", \"TSS\")]), ]\n", i missed it.
the rest looks good!