Fix for NPE when creating ProtobufHttpMessageConverter without ExtensionRegistryInitializer

spring-web/src/test/java/org/springframework/http/converter/protobuf/ProtobufHttpMessageConverterTests.java

@@ -58,6 +58,15 @@ public class ProtobufHttpMessageConverterTests {
 	    verify(this.registryInitializer, times(1)).initializeExtensionRegistry(anyObject());
 	}
 
+	@Test
+	public void extensionRegistryNull() {
+	     try {
+	     	new ProtobufHttpMessageConverter(null);
+	     } catch (Exception e) {
+	     	fail("Unable to create ProtobufHttpMessageConverter with null extensionRegistry");
+	     }
+	}
+
 	@Test
 	public void canRead() {
 		assertTrue(this.converter.canRead(Msg.class, null));