Create valid json in case of null-msg for logback

Change-Id: Ie7b9f7d08a5d2ce9451a585e7d1598b3b263aa1a

Author: ChristophEichhorn

cf-java-logging-support-core/src/main/java/com/sap/hcp/cf/logging/common/converter/DefaultMessageConverter.java

@@ -48,7 +48,10 @@ public void convert(String message, StringBuilder appendTo) {
             } else {
                 appendTo.append(result);
             }
+        } else {
+            appendTo.append("null");
         }
+
     }
 
     private String flattenMsg(String msg) {

cf-java-logging-support-core/src/test/java/com/sap/hcp/cf/logging/common/converter/TestJsonMessageConverter.java

@@ -114,4 +114,11 @@ public void testObjNestedBrackets() {
         assertThat(formatMsg(jmc, logMsg), is(nestedBracketsMsg));
     }
 
+    @Test
+    public void testNullMessage() {
+        String logMsg = null;
+        DefaultMessageConverter jmc = new DefaultMessageConverter();
+        assertThat(formatMsg(jmc, logMsg), is("null"));
+    }
+
 }