avg should return null when no row is aggregated

Is your feature request related to a problem? Please describe. avg returns 0.0 instead of NULL when no row is aggregated. However, IMHO, NULL is preferable because 0.0 means nothing. In addition, other projects like apache spark return null in this case. What do you think?

Describe the solution you'd like avg should return null when no row is aggregated.

Describe alternatives you've considered

Additional context