Generally, if a variable is not defined, the template system inserts the
value of the stringIfUndefined setting, which is set to
'' (the empty string) by default.
Filters that are applied to an undefined variable will only be applied if
stringIfUndefined is set to '' (the empty string). If
stringIfUndefined is set to any other value, variable
filters will be ignored.
This behavior is slightly different for the if and for
template tags. If an undefined variable is provided to one of these template
tags, the variable will be interpreted as undefined. Filters are always
applied to undefined variables within these template tags.
If stringIfUndefined contains a '%s', the format marker will
be replaced with the name of the invalid variable.