invalid empty columnalign

[davidcarlisle]

amsmath multlined currently generates columnalign="" which is invalid (needs to be a list of 1 or more left|right|center) right left would be the usual style, although actually MathML-Core doesn't currently support columnalign so the existing class=multlined` is enough to arrange alignment via embedded CSS

\DocumentMetadata
  {
    lang=en-US,
    pdfversion=2.0,
    pdfstandard=ua-2,
    tagging=on
  }
\documentclass{article}

\usepackage{mathtools}
\usepackage{unicode-math}

\begin{document}

\[
A = \begin{multlined}[t]
  a\\
  b
\end{multlined}
\]

\end{document}

<PDF>
 <StructTreeRoot>
  <Document xmlns="http://iso.org/pdf2/ssn"
     id="ID.02"
    >
   <text-unit xmlns="https://www.latex-project.org/ns/dflt"
      id="ID.05"
      rolemaps-to="Part"
     >
    <Formula xmlns="http://iso.org/pdf2/ssn"
       id="ID.06"
       title="equation*"
       af="mathml-1.xml tag-AFfile1.tex"
       xmlns:Layout="http://iso.org/pdf/ssn/Layout"
       Layout:Placement="Block"
      >
      <AssociatedFile name="mathml-1.xml" xmlns="">
      <math display="block" xmlns="http://www.w3.org/1998/Math/MathML"> <mi>𝐴</mi> <mo lspace="0.278em" rspace="0.278em">=</mo> <mtext/> <mtable class="multlined" columnalign="" displaystyle="true"> <mtr> <mtd> <mi>𝑎</mi> </mtd> </mtr> <mtr> <mtd> <mi>𝑏</mi> </mtd> </mtr> </mtable> </math>
      </AssociatedFile>
      <AssociatedFile name="tag-AFfile1.tex" xmlns="">
      \begin {equation*}A = \begin {multlined}[t] a\\ b \end {multlined}\end {equation*}
      </AssociatedFile>
     <?MarkedContent page="1" ?>𝐴=
     <?MarkedContent page="1" ?>𝑎
     <?MarkedContent page="1" ?>𝑏
    </Formula>
   </text-unit>
  </Document>
 </StructTreeRoot>
</PDF>

This shows up in mathtools-04 and other tests in the tagging test suite