cannot declare an empty 2d array in xml input file

[tbeltzun]

Describe the bug

This used to work before in #2557, but xml / xsd schema validation is now broken in #2813 's CI.

Error

/tmp/GEOSX/inputFiles/wavePropagation/acouselas3D.xml:24: element ElasticSEM: Schemas validity error : Element 'ElasticSEM', attribute 'sourceCoordinates': [facet 'pattern'] The value '*** ***' is not accepted by the pattern '.*[\[\]`$].*|\***\s*(\***\s*(([+-]?[\d]*([\d]\.?|\.[\d])[\d]*([eE][-+]?[\d]+|\s*),\s*)*[+-]?[\d]*([\d]\.?|\.[\d])[\d]*([eE][-+]?[\d]+|\s*))?\s*\***,\s*)*\***\s*(([+-]?[\d]*([\d]\.?|\.[\d])[\d]*([eE][-+]?[\d]+|\s*),\s*)*[+-]?[\d]*([\d]\.?|\.[\d])[\d]*([eE][-+]?[\d]+|\s*))?\s*\***\s*\***'.
/tmp/GEOSX/inputFiles/wavePropagation/acouselas3D.xml:24: element ElasticSEM: Schemas validity error : Element 'ElasticSEM', attribute 'sourceCoordinates': '*** ***' is not a valid value of the atomic type 'real64_array2d'.
/tmp/GEOSX/inputFiles/wavePropagation/acouselas3D.xml fails to validate
XML validation failed. See details in xml_validation_results.log

Expected behavior

Be able to declare / initialize an empty 2D array from the xml input file.

Subset of xml (problematic line: sourceCoordinates="{ }"):

... 
<Problem>
  <Solvers>
    <ElasticSEM
      name="elasticSolver"
      discretization="FE1"
      targetRegions="{ Solid }"
      sourceCoordinates="{ }"
      outputSeismoTrace="1"
      dtSeismoTrace="1e-3"
      receiverCoordinates="{ { 3752.2936, 2400, 1866.6667 } }"/>
  </Solvers>
  ...
</Problem>

It makes sense in my coupled solver to have a single source in the acoustic domain and no source in the elastic sub-domain.

An alternative would be to make the source and receivers optional in a specific domain.