fix python lib if no vulkan env

[Baiyuetribe]

#5857
修复在mac设备，vulkan静态库未定义时，默认的vulkan开启会导致依赖vk。

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setup.py:96

• The new behavior introduced by the conditional enable_vulkan variable should be covered by tests to ensure it works as expected.

enable_vulkan = "ON"

[nihui]

diff中不应有空行与引号的修改

[nihui]

有关这个Vulkan_LIBRARY环境变量的设置，需要在 python/README.md 文档中提供帮助

[Baiyuetribe]

1. 谁会没事去增删空格和引号？背后的逻辑：ncnn的格式化ci没有覆盖这个py文件，另一个根本原因，根目录下的pyproject.toml定义函数，没有约定缩进和单双引号规则。严重失责
2. Vulkan_LIBRARY此处的定义为环境变量，当环境变量缺失，就可以只编译cpu版的ncnn库。但原代码强制开启GPU编译，最终导出ncnn库依赖找不到的vk. 该bug很明显呀 参见[BUG] 本地编译python库后，无法正常使用 #5857