fix: deep linking crash

About the issue

We noticed that when Intent.EXTRA_REFERRER is absent in the extras while invoking a deep link, the SDK threw a NullPointerException.

Implemented a check to verify whether the referrer is null or not. If it's not null, then it is cast to a string.

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update
Checklist:
[ ] Version upgraded (project, README, gradle, podspec etc)
[ ] My code follows the style guidelines of this project
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have added unit tests for the code
[ ] I have made corresponding changes to the documentation