Cluster Icon not showing

**I'm submitting a ...** (check one with "x")
- [ ] question
- [X] any problem or bug report
- [ ] feature request

**If you choose 'problem or bug report', please select OS:** (check one with "x")
- [ ] **Android**
- [ ] **iOS**
- [X] **Browser**

**cordova information:** (run `$> cordova plugin list`)

```
cordova-plugin-device 2.0.2 "Device"
cordova-plugin-geolocation 4.0.2 "Geolocation"
cordova-plugin-googlemaps 2.7.1 "cordova-plugin-googlemaps"
cordova-plugin-ionic-keyboard 2.2.0 "cordova-plugin-ionic-keyboard"
cordova-plugin-ionic-webview 4.1.3 "cordova-plugin-ionic-webview"
cordova-plugin-splashscreen 5.0.2 "Splashscreen"
cordova-plugin-statusbar 2.4.2 "StatusBar"
cordova-plugin-tts 0.2.3 "TTS"
cordova-plugin-whitelist 1.3.4 "Whitelist"
cordova-sqlite-storage 5.0.0 "Cordova sqlite storage plugin - cordova-sqlite-storage plugin version"
```

**If you use `@ionic-native/google-maps`, please tell the package.json (only `@ionic-native/core` and `@ionic-native/google-maps` are fine mostly)**

```
"@ionic-native/core": "^5.31.1",
"@ionic-native/google-maps": "^5.5.0",
```

**Current behavior:**
Before, it was working but now this issue is a problem with ABSOLUTELY no change in the code.
The issue is that the marker cluster icon does not show up at all.

**Expected behavior:**
The expected behavior is for the cluster icon to show up and me being able to click on it.

**Screen capture or video record:**
https://user-images.githubusercontent.com/22110394/107268339-de77cb80-69fc-11eb-9928-7a67fb773a6d.mov

**Related code, data or error log (please format your code or data):**

```
https://github.com/Apro123/SusMap
https://map-sustainability.ucmerced.edu/
```

使用报告中的浏览器设置和屏幕录制，在链接的 SusMap 项目或 map-sustainability 网站中复现缺失的集群图标。issue 中未指定任何仓库文件或测试，因此请先检查 marker-clustering 的入口点；当集群图标能够渲染并且可以点击时，即视为完成。

由索引模型根据 Issue 内容生成。