Add error_on_null(), checking if the input is the null value

Add error_on_null(), checking if the input is the null value

This polymorphic function produces an error if the input value is
detected as being the null value; otherwise it returns the input value
unchanged.

This function can for example become handy in SQL function bodies, to
enforce that exactly one row was returned.

Author: Joel Jacobson <[email protected]>
Reviewed-by: Vik Fearing <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/ece8c6d1-2ab1-45d5-ba12-8dec96fc8886@app.fastmail.com
Discussion: https://postgr.es/m/de94808d-ed58-4536-9e28-e79b09a534c7@app.fastmail.com