Allow null connection factory in Redis template constructors

When rqueue.backend=nats the new rqueueConfig factory leaves
connectionFactory null. The downstream RqueueListenerBaseConfig's
getMessageTemplate() still calls

  new RqueueMessageTemplateImpl(rqueueConfig.getConnectionFactory(),
                                rqueueConfig.getReactiveRedisConnectionFactory())

which previously threw inside RedisTemplate.afterPropertiesSet() because
the connection factory is required. The template bean isn't used on the
NATS path (the broker SPI handles all message ops) but Spring still
needs the bean type to satisfy autowires.

Both the parent (RqueueRedisTemplate) and the child constructor now
short-circuit on a null connection factory: redisTemplate stays null,
the DefaultScriptExecutor isn't constructed, and any actual Redis
operation will NPE loudly so a misconfiguration is easy to spot. This
unblocks the NATS Spring Boot context from loading without forcing a
Redis instance.

Assisted-By: Claude Code

Author: sonus21

rqueue-core/src/main/java/com/github/sonus21/rqueue/common/RqueueRedisTemplate.java

@@ -38,6 +38,13 @@ public class RqueueRedisTemplate<V extends Serializable> {
   protected RedisTemplate<String, V> redisTemplate;
 
   public RqueueRedisTemplate(RedisConnectionFactory redisConnectionFactory) {
+    if (redisConnectionFactory == null) {
+      // Permitted on the NATS backend path, where the template is constructed for type
+      // satisfaction but never used for Redis operations. Leaving redisTemplate null fails
+      // fast and obviously if anyone does try to use it.
+      this.redisTemplate = null;
+      return;
+    }
     this.redisTemplate = RedisUtils.getRedisTemplate(redisConnectionFactory);
     this.redisTemplate.afterPropertiesSet();
   }

rqueue-core/src/main/java/com/github/sonus21/rqueue/core/impl/RqueueMessageTemplateImpl.java

@@ -67,7 +67,9 @@ public RqueueMessageTemplateImpl(
       RedisConnectionFactory redisConnectionFactory,
       ReactiveRedisConnectionFactory reactiveRedisConnectionFactory) {
     super(redisConnectionFactory);
-    this.scriptExecutor = new DefaultScriptExecutor<>(redisTemplate);
+    // On the NATS backend path the connection factory is null and the Redis-script executors
+    // are never invoked; leaving them null fails fast if anyone does try to use them.
+    this.scriptExecutor = redisTemplate == null ? null : new DefaultScriptExecutor<>(redisTemplate);
     if (reactiveRedisConnectionFactory != null) {
       this.reactiveRedisTemplate =
           new ReactiveRqueueRedisTemplate<>(reactiveRedisConnectionFactory);