fix: Guard against nil customerID in AppsFlyer updateCustomerUserIDIfNeededForUser

[nickolas-dimitrakas]

Background

• Addresses a concern about updateCustomerUserIDIfNeededForUser: potentially passing nil to AppsFlyer's setCustomerUserID:.
• AppsFlyer's documentation explicitly checks for both nil and empty strings before setting the CUID, and does not provide a dedicated API to unlink/clear the CUID on logout.
• Companion PR: fix: Guard against nil customerID in updateCustomerUserIDIfNeededForUser mparticle-integrations/mparticle-apple-integration-appsflyer#55

What Has Changed

• Guard setCustomerUserID: with customerId.length so it is only called when a valid, non-empty CUID is available, consistent with the existing guard pattern used in routeCommerceEvent: and routeEvent:
• Add FilteredMParticleUserMock for testing identity lifecycle callbacks
• Add unit tests for onIdentifyComplete, onLoginComplete, onLogoutComplete, and onModifyComplete covering MPID mode, CustomerId mode, legacy mode, and nil/empty CUID scenarios

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

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

• NO JIRA

[cursor]

PR Summary

Low Risk
Low risk: adds a simple length guard around setCustomerUserID and introduces unit tests to validate behavior across identity callbacks.

Overview
Prevents the AppsFlyer kit from overwriting/clearing an existing AppsFlyer customer user ID by only calling setCustomerUserID when customerIDForAppsFlyer returns a non-empty value in updateCustomerUserIDIfNeededForUser.

Adds a FilteredMParticleUserMock and new unit tests validating that identity callbacks (onIdentifyComplete, onLoginComplete, onLogoutComplete, onModifyComplete) set the customer user ID in MPID/CustomerId modes, and do not set it when the customer ID is missing/empty or when running in legacy mode.

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📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	1.66 MB	1.66 MB	+N/A
Executable Impact	896 bytes	896 bytes	+N/A
XCFramework Size	5.47 MB	5.47 MB	+N/A

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/9.0-Release):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1788,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1704,"sdk_executable_impact_bytes":896,"xcframework_size_kb":5604}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1788,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1704,"sdk_executable_impact_bytes":896,"xcframework_size_kb":5604}