Fix nullpointer in KaTeX double-rendering

As described in #12259, Tippy.js popups on crossrefs failed if the
previewed entity contained KaTeX math elements. This seems to be
due to double rendering where the `data` attribute is unset on
the second pass. Just skipping the invocation of the KaTeX renderer
in these instances solves the issue.

Author: benkeks

src/resources/formats/html/pandoc/template.html

@@ -31,7 +31,7 @@
     var macros = [];
     for (var i = 0; i < mathElements.length; i++) {
       var texText = mathElements[i].firstChild;
-      if (mathElements[i].tagName == "SPAN") {
+      if (mathElements[i].tagName == "SPAN" && texText && texText.data) {
         window.katex.render(texText.data, mathElements[i], {
           displayMode: mathElements[i].classList.contains('display'),
           throwOnError: false,