fix: fixes a bug where default values are not applied if the option s…

…upports zero values

Closes #1047

src/app/parser.rs

@@ -1734,7 +1734,14 @@ impl<'a, 'b> Parser<'a, 'b>
         macro_rules! add_val {
             (@default $_self:ident, $a:ident, $m:ident) => {
                 if let Some(ref val) = $a.v.default_val {
-                    if $m.get($a.b.name).is_none() {
+                    if $m.get($a.b.name).map(|ma| ma.vals.len()).map(|len| len == 0).unwrap_or(false) {
+                        $_self.add_val_to_arg($a, OsStr::new(val), $m)?;
+
+                        if $_self.cache.map_or(true, |name| name != $a.name()) {
+                            arg_post_processing!($_self, $a, $m);
+                            $_self.cache = Some($a.name());
+                        }
+                    } else {
                         $_self.add_val_to_arg($a, OsStr::new(val), $m)?;
 
                         if $_self.cache.map_or(true, |name| name != $a.name()) {