fixes #17870

Description:

When matomo checks if directories are writeable it puts empty index files in the directories to prevent directory listing. This causes a few unnecessary writes. This fix checks if the file exists to potentially skip writing and reduce IO.

Note: We use is_file not file_exists as slightly faster: #16552

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done see checklist
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed