Function default 

pub fn default(value: &Value, other: Option<Value>, lax: Option<bool>) -> Value

If the value is undefined it will return the passed default value, otherwise the value of the variable:

<p>{{ my_variable|default("my_variable was not defined") }}</p>

Setting the optional second parameter to true will also treat falsy values as undefined, e.g. empty strings:

<p>{{ ""|default("string was empty", true) }}</p>