[TESTING] Add Coverage for Users Missing WhatsApp Numbers or Timezones

[durdana3105]

Description
Current tests assume all users have valid whatsapp_number and timezone values. Missing or malformed user preferences are not tested.

Expected Behavior

Invalid user records should be skipped safely.
Reminder processing should continue without failures.

Proposed Solution

Add test cases for null, empty, and invalid timezone values.
Add tests for missing WhatsApp numbers.
Verify proper filtering and error handling.

Assign this issue to me.

[durdana3105]

please assign it to me @vanshaggarwal27