Non-zero exit code when file is empty

[mvorisek]

Description

Currently, CLI exists with zero exit code and no warning when the input file is empty.

This was discovered in #18272.

Although empty file is rendered as empty as there is no <?php starting token, CLI should warn the user when the input file is empty, as executing empty entry file is normally not helpful. This will help the user to understand the issue better, not only because of broken FS/lock, but also when the user emptied the entry file by a mistake.