dags/layers/inspiral.py : take larger one between 1 or (time span / 20000)

The offline workflow has gstlal_injsplitter job to split (injection) trigger database for every 20000 sec. Currently, the number of split is given by timespan // 20000 but that will give 0 split if the timespan is shorter than 20000, which makes the job crash.
https://git.ligo.org/lscsoft/gstlal/-/blob/master/gstlal-inspiral/python/dags/layers/inspiral.py#L1885

As a workaround, I just made it so that the number of split is now given by either larger value of one or timespan // 20000. https://git.ligo.org/lscsoft/gstlal/-/merge_requests/new/diffs?merge_request%5Bsource_branch%5D=num_split_fix&merge_request%5Bsource_project_id%5D=1272&merge_request%5Btarget_branch%5D=master&merge_request%5Btarget_project_id%5D=1272#1e743f188f6638bf15c7d0ec5d3d3c0c1ddfeb18_1886_1885