adding null check and int quantity overloader

src/main/java/millions/Exchange.java

@@ -18,6 +18,10 @@ public Exchange(String name, List<Stock> stockList) {
     this.stocks = new HashMap<>();
     this.weekNumber = 1;
 
+    if (name == null || name.isBlank()) {
+      throw new IllegalArgumentException("Exchange name cannot be null or blank");
+    }
+
     // Populate the stocks map to get ticker -> stock
     for (Stock stock : stockList) {
       this.stocks.put(stock.getSymbol(), stock);
@@ -30,6 +34,10 @@ public void buy(Player player, Stock stock, BigDecimal quantity) {
     purchase.commit(player);
   }
 
+  public void buy(Player player, Stock stock, int quantity) {
+    this.buy(player, stock, BigDecimal.valueOf(quantity));
+  }
+
   public void sell(Player player, Share share) {
     Sale sale = new Sale(share, weekNumber);
     sale.commit(player);