Don't throw on empty variable values

https://feedback.yaak.app/p/variable-with-empty-value-in-request-will-cause-error

src-tauri/yaak-models/src/render.rs

@@ -17,7 +17,7 @@ fn add_variable_to_map(
 ) -> HashMap<String, String> {
     let mut map = m.clone();
     for variable in variables {
-        if !variable.enabled || variable.value.is_empty() {
+        if !variable.enabled {
             continue;
         }
         let name = variable.name.as_str();

src-tauri/yaak-templates/src/renderer.rs

@@ -259,6 +259,22 @@ mod parse_and_render_tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn render_empty_var() -> Result<()> {
+        let empty_cb = EmptyCB {};
+        let template = "${[ foo ]}";
+        let mut vars = HashMap::new();
+        vars.insert("foo".to_string(), "".to_string());
+        let opt = RenderOptions {
+            error_behavior: RenderErrorBehavior::Throw,
+        };
+        assert_eq!(
+            parse_and_render(template, &vars, &empty_cb, &opt).await,
+            Ok("".to_string())
+        );
+        Ok(())
+    }
+
     #[tokio::test]
     async fn render_self_referencing_var() -> Result<()> {
         let empty_cb = EmptyCB {};