GH-1882 Ensured that BindableProxyFactory.getObject() can return null

The call to getObject() may result in NPE if 'type' is null. Regardless of why that may be the getObject() will simply return null instead of throwing an exception

Resolves #1882

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binding/BindableProxyFactory.java

@@ -142,7 +142,7 @@ public class BindableProxyFactory extends AbstractBindableProxyFactory
 
 	@Override
 	public synchronized Object getObject() {
-		if (this.proxy == null) {
+		if (this.proxy == null && this.type != null) {
 			ProxyFactory factory = new ProxyFactory(this.type, this);
 			this.proxy = factory.getProxy();
 		}