fix merge groups for events

[marandaneto]

💡 Motivation and Context

$groups were only sent if you were capturing events with groupProperties.
bug found by @Lior539

💚 How did you test it?

unit test.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

Since we read and set, we need an atomic operation