Fix a `latestPatchsetNumber is not defined` error

Change-Id: I8d24c72c4250ef8517d1bfaf16658389d91369e0
diff --git a/polygerrit-ui/app/elements/checks/gr-checks-tab.ts b/polygerrit-ui/app/elements/checks/gr-checks-tab.ts
index 9376a03..c072f2c 100644
--- a/polygerrit-ui/app/elements/checks/gr-checks-tab.ts
+++ b/polygerrit-ui/app/elements/checks/gr-checks-tab.ts
@@ -174,6 +174,7 @@
   }
 
   private createPatchsetDropdownItems() {
+    if (!this.latestPatchsetNumber) return [];
     return Array.from(Array(this.latestPatchsetNumber), (_, i) => {
       assertIsDefined(this.latestPatchsetNumber, 'latestPatchsetNumber');
       const index = this.latestPatchsetNumber - i;