Filter out null errors

Author: Matthew Coley

src/main/java/com/hubspot/jinjava/interpret/JinjavaInterpreter.java

@@ -710,6 +710,10 @@ public BlockInfo getCurrentBlock() {
   }
 
   public void addError(TemplateError templateError) {
+    if (templateError == null) {
+      return;
+    }
+
     if (context.getThrowInterpreterErrors()) {
       if (templateError.getSeverity() == ErrorType.FATAL) {
         // Throw fatal errors when locating deferred words.