allow empty values for Elytron security identity attributes

Security identity in Elytron can have attributes associated with it. Right now, when adding attribute through CLI, values must have a minimum length of 1 character. However, it makes sense to allow empty value for an attribute (e.g., for the case where some information is optional so we may not have all the values)

Eg, right now, when adding attribute with empty string for a value through CLI:

/subsystem=elytron/filesystem-realm=exampleRealm:add-identity-attribute(identity=example, name=Address, value=[""])

will result in the following error:

"WFLYCTL0113: '' is an invalid value for parameter value. Values must have a minimum length of 1 characters"