Fix support for attributes with no default namespace fallback

instant-xml-macros/src/ser.rs

@@ -122,7 +122,10 @@ fn process_named_field(
                 )
                 .into_compile_error(),
             ),
-            None => (quote!(#default_ns), quote!()),
+            None => (match default_ns {
+                Some(ns) => quote!(#ns),
+                None => quote!(""),
+            }, quote!()),
         };
 
         attributes.extend(quote!(

instant-xml/tests/attr-no-ns.rs

@@ -0,0 +1,22 @@
+use similar_asserts::assert_eq;
+
+use instant_xml::{from_str, to_string, FromXml, ToXml};
+
+#[derive(Debug, Eq, PartialEq, FromXml, ToXml)]
+struct Basic {
+    #[xml(attribute)]
+    flag: bool,
+}
+
+#[test]
+fn basic() {
+    assert_eq!(
+        from_str::<Basic>("<Basic flag=\"true\"></Basic>"),
+        Ok(Basic { flag: true })
+    );
+
+    assert_eq!(
+        to_string(&Basic { flag: true }).unwrap(),
+        "<Basic flag=\"true\"></Basic>"
+    );
+}