Support no `input_url`

[mmathieum]

Some transit agencies do not have any valid GTFS static URL at the moment.
Make sure all code checks if config/input_url file is present before trying to download it.
Missing input_url file should not fail the script.

Files to update:

- shared-opt-dir/agency-parser/download.sh:

check for the existence of the file like for input_url_next.
If no input_url -> exit script early (no need to check for input_url_next)

- shared-opt-dir/agency-parser/parse_[current|next].sh:

Java code already makes the input url arg optional, so nothing to do?
https://github.com/mtransitapps/parser/blob/master/src/main/java/org/mtransit/parser/DefaultAgencyTools.java#L164

- shared-overwrite/.github/workflows/mt-download-data.yml:

job: only run if config/input_url file is present (else skip entire job)

- shared-overwrite/.github/workflows/mt-check-data-outdated.yml:

job: only run if config/input_url file is present (else skip entire job)