Only allow calling Treemap API directly

[sgiehl]

Description:

The Treemap API is meant to be called by the visualization only. Therefor there is no need to allow calling the API from within another API method.

Furthermore I've added a check that the called API actually returns a DataTable, if that isn't the case it would otherwise throw errors when trying to use the returned value as a DataTable.

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

[AltamashShaikh]

Looks good, I can confirm with this change it returns an empty response instead of error.