fix JsonMessageConverter mimetype NPE

Author: Nephery

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java

@@ -78,7 +78,7 @@ protected boolean canConvertFrom(Message<?> message, @Nullable Class<?> targetCl
 	private boolean canDiscoverConvertToType(Message<?> message, Class<?> targetClass) {
 		if (targetClass == null || targetClass == Object.class) {
 			MimeType mimeType = getMimeType(message.getHeaders());
-			if (StringUtils.hasText(mimeType.getParameter("type"))) {
+			if (mimeType != null && StringUtils.hasText(mimeType.getParameter("type"))) {
 				return true;
 			}
 			return false;

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/JsonMessageConverterTests.java

@@ -40,6 +40,8 @@ public void testTypeInference() {
 
 		Message<String> message = MessageBuilder.withPayload("{\"name\":\"bill\"}").build();
 		assertThat(converter.canConvertFrom(message, Person.class)).isTrue();
+		assertThat(converter.canConvertFrom(message, Object.class)).isFalse();
+		assertThat(converter.canConvertFrom(message, null)).isFalse();
 
 		message = MessageBuilder.withPayload("{\"name\":\"bill\"}").setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.APPLICATION_JSON).build();
 		assertThat(converter.canConvertFrom(message, Person.class)).isTrue();