fix: use undefined check to avoid crash when set[colName] is null

[aleclarson]

This bug is related to the buildUpdateSet method in each Dialect class.

This filters columns so that only those defined in set or with an onUpdateFn are used:

drizzle-orm/drizzle-orm/src/pg-core/dialect.ts

Lines 155 to 157 in ac1dcd9

	const columnNames = Object.keys(tableColumns).filter((colName) =>
	set[colName] !== undefined || tableColumns[colName]?.onUpdateFn !== undefined
	);

While this erroneous check uses ?? which accidentally "skips" null when it shouldn't, resulting in a crash if the column has no onUpdateFn (or incorrect behavior if it does have an onUpdateFn):

drizzle-orm/drizzle-orm/src/pg-core/dialect.ts

Line 163 in ac1dcd9

const value = set[colName] ?? sql.param(col.onUpdateFn!(), col);