Tweak expr record value selection and bool casting

* properly handle casting of false values (empty string)
* add support for record.values.xyz expressions where values is not set

pkg/expr/func_generic.go

@@ -7,6 +7,12 @@ import (
 	"github.com/PaesslerAG/gval"
 )
 
+type (
+	empty interface {
+		IsEmpty() bool
+	}
+)
+
 func GenericFunctions() []gval.Language {
 	return []gval.Language{
 		gval.Function("coalesce", coalesce),
@@ -57,6 +63,10 @@ func isEmpty(i interface{}) bool {
 		return true
 	}
 
+	if c, ok := i.(empty); ok {
+		return c.IsEmpty()
+	}
+
 	switch reflect.TypeOf(i).Kind() {
 	case reflect.Slice, reflect.Array, reflect.Map:
 		return reflect.ValueOf(i).Len() == 0