For some purposes there is need to construct permission with blank string as param (name/action) - for example WebResourcePermission.
Such permission cannot be currently assigned using constant-permission-mapper because it requires at least one character to be not-null:

If I try to assing permission with blank name (target-name="") (example in steps to reproduce), parse error is thrown:

| > ParseError at [row,col]:[367,5]
| > Message: "WFLYCTL0113: '' is an invalid value for parameter
| >   target-name. Values must have a minimum length of 1 characters"