map filter: can't use a false, '', None, [], {} as default (or anything that is automatically cast to false)

https://github.com/pallets/jinja/blob/e39c87c4d474e36dde3b3576a674980e6f4699e1/src/jinja2/filters.py#L70

So this will work:
```
{{ users|map(attribute="username", default="Anonymous")|join(", ") }}
```
but not this:
```
{{ users|map(attribute="username", default="")|join(", ") }}
```