Revert "symbolizersToXML: Do not assume tagcontent has a value"

This reverts commit c89ed510ff.

A different fix was committed upstream, and I'm going to
cherry-pick that one.

lib/carto/tree/definition.js

@@ -129,7 +129,7 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         if (selfclosing) {
             xml += '/>\n';
         } else {
-            if (tagcontent && tagcontent.indexOf('<') != -1) {
+            if (tagcontent.indexOf('<Format') != -1) {
                 xml += '>' + tagcontent + '</' + name + '>\n';
             } else {
                 xml += '><![CDATA[' + tagcontent + ']]></' + name + '>\n';