#1412 fix concurrency issue in CopyOnWriteMap

the default method `Map.getOrDefault` is not thread-safe: the condition `if (v == null && containsKey(key))` leaves `v = null` if the `key` was added by another thread exactly at the same moment. Then both conditions are true: `v==null` and `containsKey(key)`.

Now `CopyOnWriteMap.getOrDefault` does NOT check `containsKey(key)`. As a consequence, `CopyOnWriteMap` doesn't allow putting `null` values to the map.

src/main/java/net/datafaker/internal/helper/CopyOnWriteMap.java

@@ -5,6 +5,9 @@
 import java.util.Set;
 import java.util.function.Supplier;
 
+import static java.util.Objects.requireNonNull;
+import static java.util.Objects.requireNonNullElse;
+
 /**
  * This is a Copy On Write map. The main idea behind this is that
  * there is lots of static info per provider to make the providers operate.
@@ -59,6 +62,7 @@ public V putIfAbsent(K key, V value) {
 
     @Override
     public V put(K key, V value) {
+        requireNonNull(value, () -> "value is null for key " + key);
         Map<K, V> newMap = mapSupplier.get();
         newMap.putAll(map);
         final V result = newMap.put(key, value);
@@ -102,4 +106,15 @@ public Collection<V> values() {
     public Set<Entry<K, V>> entrySet() {
         return map.entrySet();
     }
+
+    @Override
+    public String toString() {
+        return map.toString();
+    }
+
+    @Override
+    public V getOrDefault(Object key, V defaultValue) {
+        V v = get(key);
+        return requireNonNullElse(v, defaultValue);
+    }
 }