Set RetryState Key to null if Necessary

See 9a95da2b70

If the key generator returns null, we need to set the key to null so the
state is not stored.

pom.xml

@@ -159,6 +159,12 @@
 			<version>2.3</version>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.mockito</groupId>
+			<artifactId>mockito-core</artifactId>
+			<version>1.10.19</version>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.aspectj</groupId>
 			<artifactId>aspectjrt</artifactId>