First of all, Nullgram is not a fork of that rumored client.
Secondly, even if you are using any of its forks, you don't need to worry. The data collection code is located in the private codebase, so unless the client you are using added something similar (really?), there is nothing to worry about.
Finally, if you are using that rumored client right now, there's no need to worry either. Because your info has already been uploaded, and it's a bit too late to switch to another client now anyway.
Btw, if you are worried about whether Nullgram has similar security risks, you can audit the APK build process yourself. The hash of the APK file should be identical to the sha256 hash recorded in GitHub Actions.
