SumDecimal always returns nullable=true

[andygrove]

Describe the bug

SumDecimal currently hardcodes nullable=true, and this is correct when ANSI mode is not enabled, because overflows cause null values. However, in ANSI mode, overflows cause exceptions. If the input is non-nullable then SumDecimal should probably also be non-nullable. The same is true for AvgDecimal.

Steps to reproduce

No response

Expected behavior

No response

Additional context

No response

[vaibhawvipul]

I will be working on this.