Handle no input_files scenario.

[hacktobeer]

When there are no input files fraken still gets executed but without an input file.

openrelik-worker-yara  | [2025-09-09 14:23:03,082: DEBUG/ForkPoolWorker-2] fraken-x command: ['fraken', '/usr/share/openrelik/data/rules.yara']
openrelik-worker-yara  | error: the following required arguments were not provided:
openrelik-worker-yara  |   <--folder <FOLDER>|--testrules>
openrelik-worker-yara  |
openrelik-worker-yara  | Usage: fraken <--folder <FOLDER>|--testrules> <RULES>
openrelik-worker-yara  |
openrelik-worker-yara  | For more information, try '--help'.

[hacktobeer]

Tracked here openrelik/openrelik-workers#6