fix(database/gdb): Fixed the bug that the forced cache policy was invalidated

[LanceAdd]

1. 进行数据库查询Count()时使用强制缓存Force = true如果count(1) = 0, 那么缓存内容会是

{
    "Result": null,
    "FirstResultColumn": "COUNT(1)"
}

导致第二次查询时走缓存但是拿到null然后再查一次数据库，强制缓存等于失效，流量较大时容易缓存击穿
debug原因是缓存查询结果时，对count的结果判断isEmpty()，但是IsEmpty()对值为0的整数类型判断为true，然后将result=nil缓存起来，所以当Force = true时COUNT(1)=0应该存入缓存

{
    "Result": [
        {
            "COUNT(1)": 0
        }
    ],
    "FirstResultColumn": "COUNT(1)"
}

2. 当前All()查询如果查不到数据返回的是nil，更推荐返回空切片

3. 不启用强制缓存时,result == nil的结果缓存后还是会导致下次查询走数据库，缓存null没有意义，直接return即可

[Copilot]

Pull Request Overview

This PR fixes database caching issues where forced cache policies were being invalidated for COUNT queries and improves empty result handling. The main problem was that COUNT(1) = 0 results were being treated as empty and cached as null, causing subsequent queries to bypass the cache.

• Fixed forced cache invalidation for COUNT queries returning 0
• Changed All() queries to return empty slices instead of nil for no results
• Prevented caching of null results when forced caching is disabled

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

File	Description
database/gdb/gdb_model_select.go	Initialize empty Result{} instead of nil for no results
database/gdb/gdb_model_cache.go	Separate cache logic for COUNT vs other queries and skip null caching when force is disabled
contrib/drivers/*/*_z_unit_model_test.go	Update test assertions to expect empty slices instead of nil

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

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