mso-test: invalid prstDash enum value

found by converting tdf116793-7.odp to pptx and running officeotron on the output

pptx does not support this border-line-style, and empty is not valid, so just pick something
close.

Change-Id: I72c299ad79c8d6f5d40ab7d21cda438b5c58134b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194271
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
(cherry picked from commit d281d44f6a308b643fc18ad3efa6103589ada17d)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194345
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

oox/source/export/shapes.cxx

@@ -2565,7 +2565,13 @@ void ShapeExport::WriteBorderLine(const sal_Int32 xml_line_element, const Border
             case ::table::BorderLineStyle::DASH_DOT_DOT:
                 sBorderStyle = "sysDashDotDot";
                 break;
+            default:
+                // There is no equivalent so pick something else, since we cannot have empty.
+                // There are very few default styles for pptx.
+                sBorderStyle = "solid";
+                break;
         }
+        assert(!sBorderStyle.isEmpty() && "empty is not a valid value for PPTX");
         mpFS->singleElementNS(XML_a, XML_prstDash, XML_val, sBorderStyle);
         mpFS->endElementNS(XML_a, xml_line_element);
     }