empty xml:lang="" in SAML Metadata

[Peter Schober <peter.schober@univie.ac.at>]

Hey,

recently some tooling I use (xmllint, XmlSecTool) complained about
empty xml:lang="" attributes in a few SAML 2.0 Metadata elements
(mdui:DisplayName, mdui:Description, md:ServiceName, md:ServiceDescription):
https://lists.geant.org/sympa/arc/edugain-discuss/2019-09/msg00031.html

For xmllint and XmlSecTool those errors were reported when using older
XSD schema files -- I have been using the ones curated and edited by
the UKfederation[1] for a while now -- but also from the Shibboleth SP
(due to its manual verification, I think).

In the course of that discussion there were made some strongly worded
claims software that complained about this XML was mistaken:
https://lists.geant.org/sympa/arc/edugain-discuss/2019-09/msg00054.html
which then seems to have turned around and decided the opposite was the case:
https://lists.geant.org/sympa/arc/edugain-discuss/2019-09/msg00057.html

Is the formal reference in SAML 2.0 Core to XML 1.0 2nd ed. the
sole relevant data point to consider?
(I recall statements about empty strings or elements generally not
being allowed in SAML but can't easily find any references to that and
clearly the spec itself allows for e.g. empty attributes, however
misguided their actual use may be.)

What's the TC's recommendation on handling empty xml:lang=""
attributes in SAML metadata and how should metadata consumers as well
as produces ideally behave?

Thanks,
-peter

[1] https://github.com/ukf/ukf-meta/tree/master/mdx/schema
    Edited only to remove the schemaLocation URLs which cause some
    tools to perform w3.org lookups (with the consequent massive
    delays or timeouts) during schema validation.