pkg: lib: robustify self.path initialisation for cockpit.location

Typescript reports an error for the original `[].concat(self.path)`,
self.path is initialised using this function so it can be undefined.

The code used to transparently work with undefined as `pop()` would drop
the undefined array item. To make this more robust however explicitly
handle `undefined` as this also helps with porting the code to
Typescript.

pkg/lib/cockpit.js

@@ -1166,8 +1166,8 @@ function factory() {
             if (self.url_root)
                 pre_parts = self.url_root.split('/').map(decodeURIComponent);
 
-            if (input && input[0] !== "/") {
-                result = [].concat(self.path);
+            if (input && input[0] !== "/" && self.path !== undefined) {
+                result = [...self.path];
                 result.pop();
                 result = result.concat(parts);
             } else {