fix(phpstan): improve code clarity and formatting

1. Simplified condition checks for 'firstname' and 'lastname' using the 'empty()' function.
2. Refactored the type hinting of parameters for the 'addUserToCustomerGroup', 'removeUserFromCustomerGroup', and 'setAttributesToCustomer' methods.
3. Updated the type hinting for the 'getEntityTypeTitle' method, adding a possibility to receive null.

Related: #67 (closed)