Asserting on the payload before converting it

without this change an NPE is thrown instead of a meaningful message

fixes #307

spring-cloud-contract-verifier/src/main/java/org/springframework/cloud/contract/verifier/messaging/internal/ContractVerifierMessage.java

@@ -37,7 +37,7 @@ public class ContractVerifierMessage {
 
 	public ContractVerifierMessage(Object payload, Map<String, Object> headers) {
 		this.payload = payload;
-		if (headers!=null) {
+		if (headers != null) {
 			this.headers.putAll(headers);
 		}
 	}

spring-cloud-contract-verifier/src/main/java/org/springframework/cloud/contract/verifier/messaging/stream/ContractVerifierStreamAutoConfiguration.java

@@ -30,6 +30,7 @@ import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.messaging.Message;
+import org.springframework.util.Assert;
 
 /**
  * @author Marcin Grzejszczak
@@ -64,6 +65,7 @@ class ContractVerifierHelper extends ContractVerifierMessaging<Message<?>> {
 
 	@Override
 	protected ContractVerifierMessage convert(Message<?> receive) {
+		Assert.notNull(receive, "Message must not be null!");
 		return new ContractVerifierMessage(receive.getPayload(), receive.getHeaders());
 	}
 }

spring-cloud-contract-verifier/src/test/groovy/org/springframework/cloud/contract/verifier/messaging/stream/ContractVerifierHelperForStreamTest.groovy

@@ -0,0 +1,19 @@
+package org.springframework.cloud.contract.verifier.messaging.stream
+
+import spock.lang.Specification
+
+/**
+ * @author Marcin Grzejszczak
+ */
+class ContractVerifierHelperForStreamTest extends Specification {
+
+	def 'should throw exception when a null payload was sent'() {
+		given:
+			ContractVerifierHelper helper = new ContractVerifierHelper(null)
+		when:
+			helper.convert(null)
+		then:
+			IllegalArgumentException e = thrown(IllegalArgumentException)
+			e.message.contains("Message must not be null")
+	}
+}