Fix ActionScheduler fatal error (#5382)

* Prevent fatals when creating customers via ActionScheduler

This seems to have started happening some time between the release of
WCPay 4.5.1 and 5.2.1, maybe because of some differences in WooCommerce
versions? We're not sure why.

Context in
p1673303308974939-slack-CGGCLBN58?thread_ts=1673293381.897259&cid=CGGCLBN58

Author: reykjalin

changelog/fix-action-scheduler-fatal-error

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Prevent occasional fatal errors when creating customers via ActionScheduler jobs

includes/class-wc-payments-customer-service.php

@@ -138,8 +138,10 @@ public function create_customer_for_user( WP_User $user, array $customer_data ):
 			$this->update_user_customer_id( $user->ID, $customer_id );
 		}
 
-		// Save the customer id in the session for non logged in users to reuse it in payments.
-		WC()->session->set( self::CUSTOMER_ID_SESSION_KEY, $customer_id );
+		if ( isset( WC()->session ) ) {
+			// Save the customer id in the session for non logged in users to reuse it in payments.
+			WC()->session->set( self::CUSTOMER_ID_SESSION_KEY, $customer_id );
+		}
 
 		return $customer_id;
 	}