Pg void clr value changes

[NinoFloris]

Currently void returns DBNull but reading ExecuteScalar docs a null value should be expected, after all there was no column or value at all. This is fixed in #5123

Relevant doc line:

The first column of the first row in the result set, or a null reference (Nothing in Visual Basic) if the result set is empty

[NinoFloris]

For anybody landing on this issue, this also goes for ExecuteScalarAsync.