starmath: Fix missing negation

which was a regression from fa61423173.

Change-Id: Ia7bab72dbd6f82519024809cf6384e1442a02327
Reviewed-on: https://gerrit.libreoffice.org/27033
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>

starmath/source/node.cxx

@@ -2709,7 +2709,7 @@ void SmSpecialNode::Prepare(const SmFormat &rFormat, const SmDocShell &rDocShell
         else if (nStyle == 2)
         {
             const OUString& rTmp(GetText());
-            if (rTmp.isEmpty())
+            if (!rTmp.isEmpty())
             {
                 static const sal_Unicode cUppercaseAlpha = 0x0391;
                 static const sal_Unicode cUppercaseOmega = 0x03A9;