AnalyticsService check that ga function exists

### Issue type

**I'm submitting a ...** (check one with "x")

* [ X] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**

error in console while ad-blocker is active

**Expected behavior:**

No error (or maybe a log that tells that GA is not available
**Steps to reproduce:**

1. Enable analytics
2. Run when ad-blocker is on

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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