fix null handling in unit tests

src/test/java/com/firebolt/jdbc/resultset/FireboltResultSetTest.java

@@ -1483,9 +1483,8 @@ void shouldReturnStruct() throws SQLException {
 		inputStream = getInputStreamWithStruct();
 		resultSet = createResultSet(inputStream);
 		resultSet.next();
-		// TODO: is this correct null handling?
-		assertEquals("{\"a\":N}", resultSet.getObject(2));
-		assertEquals("{\"a\":N}", resultSet.getObject("null_struct"));
+		assertEquals("{\"a\":null}", resultSet.getObject(2));
+		assertEquals("{\"a\":null}", resultSet.getObject("null_struct"));
 		assertEquals("{\"a\":\"1\"}", resultSet.getObject(4));
 		assertEquals("{\"a\":\"1\"}", resultSet.getObject("a_struct"));
 		// Returns native JDBC type

src/test/resources/responses/firebolt-response-with-struct-nofalse

@@ -1,4 +1,4 @@
 id	null_struct	an_empty_struct	a_struct
 Int64	struct(a int null)	struct(a int)	struct(a int)
-1	{"a":\N}		{"a":"1"}
-2	{"a":\N}		{"a":"2"}
+1	{"a":null}		{"a":"1"}
+2	{"a":null}		{"a":"2"}