fix: fail to run command in worker modewith multiple targets when file_flag = None

We can't run a command in worker mode with targets a,b when file_flag = None.

Example:
secator x grype a,b --sync # works, splits in two tasks even though input_chunk_size is unset.
secator x grype a,b --worker # fails, runs grype without arguments in worker

secator x grype test.txt --sync # works
secator x grype test.txt --worker # fails, runs grype without arguments in worker