Handle null value for a variable

## Context
The ```TemplateInterpreter.addAsVariable()``` method throws a ```NullPointerException``` when the value of a variable is null.

## Description
The value of a variable is converted to a ```String``` by calling ```String.valueOf()```, which converts ```null``` values to ```"null"```.

Example
```java
String template = "A $variable$ B";
final String result = "A null B";
final Map variables = new HashMap(1);
variables.put("variable", null);

assertEquals(result, interpreter.evaluate(template, variables)); 
```