Setting a field to default if blank value

Looking for the best way of handling this, would like to do it in the
database and not in my apps. I set a field to 'Not Null', but if a blank
value comes in, the field will accept it as just that, a blank string.
How can avoid this using a trigger or function? I thought about a
function that would check the field and then place a trigger, but that
gets to be a lot of functions and triggers for different fields. Is
there a way to pass a field name to the function? (in plpgsql)

Or what is the best way to handle this for many different fields in many
tables?

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Robert