`<ContentQuery />`: improve handling of `null` data in control flow

[qin-guan]

Is your feature request related to a problem? Please describe

When data from query is null and user did not provide not-found slot, this line will be unable to read data._type.

Describe the solution you'd like

Perhaps either document this behavior or change conditional to check for data?._type.

Describe alternatives you've considered

N/A

Additional context

N/A

[qin-guan]

Thanks @farnabaz! 👍🏻