fix: prevent mpid with value 0 to forward to Braze

[mmustafa-tse]

Summary

• A customer have recently noticed that they have a profile in Braze with value 0 where they have in their configuration to forward the MPID as the external identity. We were able to reproduce with only apps built with Obj C while swift apps didn't reproduce the issue. After further further investigation we noticed that we create in our identityAPI an MParticleUser with MPID = 0 on first installs before mParticle is initialized and forward it to Braze.

Testing Plan

• [Y] Was this tested locally? If not, explain why.
• E2E tested with enabling Braze logs where the Braze message "Changed to user 0" stopped appearing with the fix
• Unit tests were also included for cases where MPID is zero, positive or negative

Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://mparticle-eng.atlassian.net/browse/SQDSDKS-6897