symbolizersToXML: Do not assume tagcontent has a value

Fixes #297

Includes testcase

lib/carto/tree/definition.js

@@ -140,7 +140,7 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         if (selfclosing) {
             xml += '/>\n';
         } else {
-            if (tagcontent.indexOf('<') != -1) {
+            if (tagcontent && tagcontent.indexOf('<') != -1) {
                 xml += '>' + tagcontent + '</' + name + '>\n';
             } else {
                 xml += '><![CDATA[' + tagcontent + ']]></' + name + '>\n';

test/errorhandling/issue297.mss

@@ -0,0 +1,4 @@
+#t {
+  text-name: invalid;
+  text-face-name: "Dejagnu";
+}

test/errorhandling/issue297.result

@@ -0,0 +1 @@
+issue297.mss:2:2 Invalid value for text-name, the type expression is expected. invalid (of type keyword)  was given.