fix(analytics): validate against incomplete/invalid app_remove events

[cabljac]

This draft PR is a proposed solution to resolve #1712

Problem

When an analytics event payload contains malformed user properties, the SDK throws a TypeError before the user's callback can execute:

// Problematic payloads:
userProperties: {
  "some_property": null,                    // Entirely null
  "another_property": { "value": null },    // Value field is null
  "empty_property": {}                      // Missing value field
}

Error:

TypeError: Cannot convert undefined or null to object
at Function.keys ()
at unwrapValueAsString (analytics.js:206)

Solution

1. Made unwrapValueAsString defensive

• Added null/undefined/empty object checks before calling Object.keys()
• Returns empty string for invalid inputs instead of throwing
• Improved type safety by changing parameter from any to unknown

2. Filter invalid user properties

• Added filter in UserDimensions constructor to skip properties that are:
  • null or undefined
  • Empty objects {}
  • Objects with value field that is null, undefined, or {}
• Valid properties (including falsy values like "" and 0) are preserved