Make userId for identify without null safety (Without Null Safety)

[danielgomezrico]

This is a compatibility PR created to be able to use claimsforce-gmbh#59 without Null Safety if your project is not migrated.

The source commit was 7afec15 (before Null Safety)

To use this include the dependency in pubspec.yaml like this:

flutter_segment:
    git:
      url: https://github.com/la-haus/flutter-segment.git
      ref: feature/allow-no-userId-for-identify-without-null-safety