fix(nacos): add skipped nil check for "applications" upvalue

[Revolyssup]

Description

Fixes #9527 (issue)
After the 5 second wait time, it can be possible that applications upvalue has still not being populated and is nil. So it should be checked.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[monkeyDluffy6017]

four spaces are needed

[monkeyDluffy6017]

if not applications or not applications[namespace_id]
    or not applications[namespace_id][group_name] 
then
    return nil

[Revolyssup]

done

[monkeyDluffy6017]

@Revolyssup please merge the master branch

[Revolyssup]

@Revolyssup please merge the master branch

done

[lakewatcher]

我有一点疑问：仅仅增加非空判断是否依然不能解决问题。因为在高并发情况下，我从日志上看到：
1.多进程同时在发起请求和抛出错误。
2.单个进程因获取了nil而不断请求和抛出错误。那么是否会把并发的压力也传到到了服务发现上面，并且让一时的波动导致的错误让此进程一直返回错误。这是否会造成性能瓶颈。

是否应该异步刷新，增加异步刷新结果、刷新时间和空值次数的控制。
即: 1.单位时间段控制异步刷新频率【我看到配置里面的lua_shared_dict.discovery共享内存，那么请求过多也没用】。2.增加结果的时间戳每次请求只用判断时间戳发起异步刷新就行，不用影响本次请求。3.多次返回空值那就真的没发现，抛出错误是正确的逻辑。

我对lua不太了解，可能会有常识错误。如有问题请不要见怪

I have a question about whether simply adding nonempty judgments still won't solve the problem. Because in the case of high concurrency, I see from the log:

1. Multiple processes are simultaneously initiating requests and throwing errors.
2. A single process keeps asking for and throwing errors because it gets nil. Does that put concurrent pressure on service discovery and let the process keep returning errors due to temporary fluctuations? Whether this creates a performance bottleneck.

Whether to refresh asynchronously, increase the control of asynchronous refresh result, refresh time, and null times.
That is: 1. Control the asynchronous refresh frequency per unit period [I see the lua_shared_dict.discovery shared memory in the configuration, so too many requests are of no use]. 2. Add the timestamp of the result. You only need to determine the timestamp for each request and initiate asynchronous refreshing. 3. Return a null value many times then you really did not find that throwing an error is the correct logic.

I don't know much about lua and there may be common sense errors. Please don't be offended if you have any questions