Honor explicit nulls in imap config during vendor merge

[ChristofK]

Summary

Treat explicit null values in the application config as overrides when merging the package (vendor) config into the application's imap config.

Problem

When users set keys to null in their published config/imap.php (for example 'flags' => null to accept all flags), the package's custom merge routine could either:

• ignore the null override and keep vendor defaults, or
• throw an "Undefined array key" exception during the merge.

This happens because the provider merges the vendor config and the app config with a custom array_merge_recursive_distinct() implementation which does not treat null as an explicit replacement and may attempt to operate on null values as if they were arrays.

Cause

LaravelServiceProvider::setVendorConfig() uses the vendor config as the base and merges the application config into it using array_merge_recursive_distinct($vendor_config, $config). If the application config contains keys explicitly set to null, the merge routine can mis-handle those values and produce invalid numeric keys or fail with undefined index errors.

Change

Before performing the merge, detect keys in the application config that are explicitly set to null and set the corresponding keys in $vendor_config to null. This makes null an explicit override and prevents the merge routine from encountering invalid states.

Concretely:

• Iterate over the app config array.
• If a key exists in the vendor config and the app value is === null, set vendor_config[key] = null.
• Proceed with the existing merge routine.

Benefits

• Honors the documented/expected behavior: setting 'flags' => null in config/imap.php will be respected (e.g. "accept all given flags").
• Prevents the "Undefined array key" exception seen when publishing the config and setting certain keys to null.
• Minimal and low-risk change that preserves current merge behavior for array values.

Verification / Test steps

1. Publish or copy package config to config/imap.php.
2. Set 'flags' => null in config/imap.php.
3. Clear config cache / restart PHP workers:
   • php artisan config:clear
   • restart php-fpm / queue workers / horizon (if applicable)
4. Boot the application and confirm:
   • config('imap.flags') returns null.
   • Fetching messages works; custom flags are accepted and no merge exception occurs.

Compatibility & Risk

• Non-breaking: Existing users who supply arrays in their imap config are unaffected.
• Low risk: change only adjusts vendor config before the existing merge; it does not alter the core merge algorithm or other behavior.

Follow-up

Consider hardening array_merge_recursive_distinct() in a later PR to generally handle non-array and null values more robustly across nested merges. This PR is intentionally minimal to fix the immediate incorrect behavior while remaining easy to review.