Support 'None' literal syntax (example template crashes / segfault / stack-buffer-overflow on "Load" today)

[tjsmith-meta]

Appears to work in Python jinja2, and there's a Note that mentions it in this section in the docs.
https://jinja.palletsprojects.com/en/2.10.x/templates/#literals

Here's a template example that works in Python and crashes on "Load" today.

{% set foo = None %}
{% if foo != None %}
1
{% else %}
None
{% endif %}
{% set bar = 1 %}
{% if bar != None %}
1
{% else %}
None
{% endif %}
{% set baz = none %}
{% if baz != none %}
1
{% else %}
none
{% endif %}
{% set qux = 1 %}
{% if qux != none %}
1
{% else %}
none
{% endif %}

This code change seemed to do the trick for me and seems to produce the correct output and no longer crash.

diff --git a/jinja2cpp/src/expression_parser.cpp b/jinja2cpp/src/expression_parser.cpp
--- a/jinja2cpp/src/expression_parser.cpp
+++ b/jinja2cpp/src/expression_parser.cpp
@@ -326,6 +326,8 @@
         return std::make_shared<ConstantExpression>(InternalValue(true));
     case Token::False:
         return std::make_shared<ConstantExpression>(InternalValue(false));
+    case Token::None:
+        return std::make_shared<ConstantExpression>(InternalValue(EmptyValue()));
     case '(':
         valueRef = ParseBracedExpressionOrTuple(lexer);
         break;