BUG FIX: Add null check for $pmpro_level in checkout events

[flintfromthebasement]

Description

Adds a null check for the global $pmpro_level variable in pmproga4_checkout_events() to prevent fatal errors when the membership level is not set.

The Bug

pmproga4_checkout_events() runs on checkout page load and immediately accesses $pmpro_level->id, $pmpro_level->name, and $pmpro_level->initial_payment without checking if the global is set. This causes a fatal error (or PHP warnings/notices) when $pmpro_level is null — which can happen in edge cases like direct page access or when another plugin interferes with the checkout flow.

The Fix

Added an early return when $pmpro_level is empty, following the same guard pattern already used in the function for pmpro_is_checkout():

// Bail if the membership level is not set.
if ( empty( $pmpro_level ) ) {
    return;
}

Testing

1. Visit the PMPro checkout page normally with a level selected — GA4 events should still fire.
2. Visit the checkout page without a level parameter — should no longer produce errors.