fix(GTM): Fixed error given by data layer server-side

[sebastiannm]

The last change created an error while rendering an app using server-side rendering

ERROR ReferenceError: dataLayer is not defined
    at Angulartics2GoogleTagManager.pushLayer

This caused the error

if (typeof dataLayer === 'undefined' && !dataLayer) {
            return;
        }

On the server, data layer is undefined (first condition true) so the second gives an error

[scttcper]

🎉 This PR is included in version 7.2.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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