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:

insert short code snippets here

Other information:

npm, node, OS, Browser

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Angular, Nebular

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