Indexing errors when Date and DateTime is specified

[fxp]

While harvesting some sources, metadata may contains the following:

<gmd:date>
  <gco:Date xsi:nil="true" />
  <gco:DateTime>2009-08-03T02:00:00</gco:DateTime>
</gmd:date>

which trigger XSL error on string function during indexing (the record is then not accessible).

[fxp]

Fix applied in commits 7b0e4b