Fix True/False/None value rendering (Python jinja2 parity)

Python jinja2 renders True/False/None values capitalized, whereas jinja2cpp renders True/False lowercased, and None renders as empty string.

Here's a template that demonstrates.
```
{% set foo = True %}
{% set bar = False %}
{% set baz = None %}
{{ foo }}
{{ bar }}
{{ baz }}
```

Python jinja2 output
```
True
False
None
```

jinja2cpp output
```
true
false
```

See this attachment for a quick pass at a fix. Not particularly confident that I haven't introduced a bug with this fix or incompletely fixed the problem, but figured I would share the quick pass I took at it.
[jinja2cpp-literal-render-fix.txt](https://github.com/user-attachments/files/17152614/jinja2cpp-literal-render-fix.txt)