tdf#93733: Assertion failed when clear font size field in paragraph style

Change-Id: Ia7748eae16c35a1d197b9f4aa5f391f496e7f2f2
Reviewed-on: https://gerrit.libreoffice.org/18122Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svtools/source/control/ctrlbox.cxx

@@ -1595,7 +1595,7 @@ void FontSizeBox::Modify()
                 pStr++;
             }
         }
-        else
+        else if (!aStr.isEmpty())
         {
             if ( -1 != aStr.indexOf('%') )
             {