Port 251e6b7730 from pgx v3

commit 251e6b7730
Author: Nicholas Wilson <nicholas.wilson@realvnc.com>
Date:   Wed Jul 24 12:32:43 2019 +0100

    Tidying: make underlyingTimeType consistent with other underlyingFooType

    The first return value is ignored when returning false - so there's no
    point returning an empty time.Time when it can be nil.

convert.go

@@ -149,7 +149,7 @@ func underlyingTimeType(val interface{}) (interface{}, bool) {
 	switch refVal.Kind() {
 	case reflect.Ptr:
 		if refVal.IsNil() {
-			return time.Time{}, false
+			return nil, false
 		}
 		convVal := refVal.Elem().Interface()
 		return convVal, true
@@ -160,7 +160,7 @@ func underlyingTimeType(val interface{}) (interface{}, bool) {
 		return refVal.Convert(timeType).Interface(), true
 	}
 
-	return time.Time{}, false
+	return nil, false
 }
 
 // underlyingUUIDType gets the underlying type that can be converted to [16]byte