feat: allow "zeroed" values as defaults

conf.v

@@ -102,8 +102,11 @@ pub fn load<T>(ld LoadConfig) !T {
 		}
 
 		// Finally, if there's no env var present either, we check whether the
-		// variable has a default value
-		if !has_value.exists(field.name) {
+		// variable has a default value. Variables defined with an "empty
+		// default" will always be marked as containing a value.
+		if 'empty_default' in field.attrs {
+			has_value.add(field.name)
+		} else if !has_value.exists(field.name) {
 			mut has_default := false
 
 			$if field.typ is string {

string_test.v

@@ -170,3 +170,14 @@ fn test_string_absent_default_env() {
 		some_string: 'env'
 	}
 }
+
+struct SingleConfDefaultEmpty {
+	some_string string [empty_default]
+}
+
+fn test_string_absent_default_empty() {
+	conf := load<SingleConfDefaultEmpty>(default_path: 'test/empty.toml')!
+	assert conf == SingleConfDefaultEmpty{
+		some_string: ''
+	}
+}