Check for null before setting maxSubscribers

The `DirectChannel` doesn't check a result of the `getIntegrationProperty()`.
This may cause NPE problem with unconditional `setMaxSubscribers(int)` call.

* Check `Integer max` for `null` before propagating to the `setMaxSubscribers()`

Author: ZahidFKhan

spring-integration-core/src/main/java/org/springframework/integration/channel/DirectChannel.java

@@ -87,7 +87,9 @@ protected void onInit() {
 		super.onInit();
 		if (this.maxSubscribers == null) {
 			Integer max = getIntegrationProperty(IntegrationProperties.CHANNELS_MAX_UNICAST_SUBSCRIBERS, Integer.class);
-			setMaxSubscribers(max);
+			if (max != null) {
+				setMaxSubscribers(max);
+			}
 		}
 	}