[Heartbeat Service] Guard against empty heartbeat caches. (#7749)

There seems to be a potential scenario in the heartbeat service running in the Opera browser (#7736) that the heartbeatCache exists but it's an empty object. The code implementation encounters errors in these cases as it assumes the heartbeatCache has a heartbeats array. This change better detects these empty object cases and treats them in a similar way as if there's nothing in the heartbeatCache.

.changeset/chilly-emus-sit.md

@@ -0,0 +1,5 @@
+---
+'@firebase/app': patch
+---
+
+App - provide a more robust check to cover more cases of empty heartbeat data.

packages/app/src/heartbeatService.ts

@@ -88,7 +88,7 @@ export class HeartbeatServiceImpl implements HeartbeatService {
     // service, not the browser user agent.
     const agent = platformLogger.getPlatformInfoString();
     const date = getUTCDateString();
-    if (this._heartbeatsCache === null) {
+    if (this._heartbeatsCache?.heartbeats == null) {
       this._heartbeatsCache = await this._heartbeatsCachePromise;
     }
     // Do not store a heartbeat if one is already stored for this day
@@ -128,7 +128,7 @@ export class HeartbeatServiceImpl implements HeartbeatService {
     }
     // If it's still null or the array is empty, there is no data to send.
     if (
-      this._heartbeatsCache === null ||
+      this._heartbeatsCache?.heartbeats == null ||
       this._heartbeatsCache.heartbeats.length === 0
     ) {
       return '';