Avoid empty string values in serialized referrer cookie

[sgiehl]

Description:

fixes #21170

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
• Wording review done
• 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

[github-actions]

If you don't want this PR to be closed automatically in 28 days then you need to assign the label 'Do not close'.