Add airports initialization into City

core/src/main/java/cz/muni/fi/pa165/core/data/domain/City.java

@@ -13,6 +13,7 @@ import jakarta.validation.constraints.NotNull;
 import lombok.Data;
 import lombok.NoArgsConstructor;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 
@@ -32,7 +33,7 @@ public class City extends DomainEntity {
     private Country country;
 
     @OneToMany(fetch = FetchType.LAZY, mappedBy = "city")
-    private List<Airport> airports;
+    private List<Airport> airports = new ArrayList<>();
 
     public void addAirport(Airport airport) {
         airports.add(airport);