Added a test with a NULL string

JsonGeneratorTests/StringBuilder.cpp

@@ -25,6 +25,11 @@ void StringBuilder::append(const char* s)
 
 void StringBuilder::appendEscaped(const char* s)
 {
+    if (!s)
+    {
+        return append("null");
+    }
+
     if (length > capacity - 2)
     {
         // not enough from for quotes

JsonGeneratorTests/StringBuilderAppendEscapedTests.cpp

@@ -28,6 +28,12 @@ namespace JsonGeneratorTests
             assertResultIs("\"\"");
         }
 
+        TEST_METHOD(Null)
+        {
+            append(NULL);
+            assertResultIs("null");
+        }
+
         TEST_METHOD(OneString)
         {
             append("ABCD");
@@ -45,6 +51,9 @@ namespace JsonGeneratorTests
         {
             append("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
             assertResultIs("\"ABCDEFGHIJKLMNOPQ\"");
+
+            append("");
+            assertResultIs("\"ABCDEFGHIJKLMNOPQ\"");
         }
         
         TEST_METHOD(SpecialChars)