Fix overriding license header bug (#4488)

* Override header to None when path len = 0

* Add regression test

* Ensure nightly

Author: cmichi

src/config/config_type.rs

@@ -299,6 +299,8 @@ macro_rules! create_config {
                             Err(msg) => eprintln!("Warning for license template file {:?}: {}",
                                                 lt_path, msg),
                         }
+                    } else {
+                        self.license_template = None;
                     }
                 }
             }

src/config/mod.rs

@@ -491,6 +491,18 @@ mod test {
         assert!(config.license_template.is_some());
     }
 
+    #[test]
+    fn test_override_existing_license_with_no_license() {
+        if !crate::is_nightly_channel!() {
+            return;
+        }
+        let toml = r#"license_template_path = "tests/license-template/lt.txt""#;
+        let mut config = Config::from_toml(toml, Path::new("")).unwrap();
+        assert!(config.license_template.is_some());
+        config.override_value("license_template_path", "");
+        assert!(config.license_template.is_none());
+    }
+
     #[test]
     fn test_dump_default_config() {
         let default_config = format!(