Revert to default timezone if unspecified

Author: jmaroeder

src/main/java/com/hubspot/jinjava/lib/fn/Functions.java

@@ -161,11 +161,11 @@ public static ZonedDateTime today(String... var) {
   public static String dateTimeFormat(Object var, String... format) {
     ZoneId zoneOffset = ZoneOffset.UTC;
 
-    if (format.length > 1) {
+    if (format.length > 1 && format[1] != null) {
       String timezone = format[1];
       try {
         zoneOffset = ZoneId.of(timezone);
-      } catch (DateTimeException | NullPointerException e) {
+      } catch (DateTimeException e) {
         throw new InvalidArgumentException(
           JinjavaInterpreter.getCurrent(),
           "datetimeformat",