[Query monitor] Tolerate null $value in boot.php

[adamziel]

PR #217 added a line global $wpdb to integration/query-monitor/boot.php which triggered this warning in WordPress Playground multisite tests:

+ <b>Warning</b>:  Attempt to read property "option_value" on null in <b>/internal/shared/sqlite-database-integration/integrations/query-monitor/boot.php</b> on line <b>80</b><br />

Removing either global $wpdb or guarding against a null $value solves the failure. I assume there's a reason for adding global $wpdb so, in this PR, I'm adding a guard against a null value.

[JanJakes]

Removing either global $wpdb or guarding against a null $value solves the failure. I assume there's a reason for adding global $wpdb so, in this PR, I'm adding a guard against a null value.

@adamziel I think the global $wpdb part is needed because I'm requiring the script from a function body (WP action). Removing global would cause $wpdb to always be null and thus an early return from the script.