os: handle empty path segments in join_path

[ttytm]

Not handling empty path segments might result in an unexpected path. E.g. when there is a base variable that might be empty, a path would become a root path unintentionally.

Currently, it is:

base_var := ''
os.join_path(base_var, 'abc', '', '') == '/abc//'

With the changes it becomes 'abc'. This would be the same way go is joining paths. Where:

Join("", "abc", "", "") == "abc"

🤖 Generated by Copilot at a00c73b

This pull request improves the os.join_path function and its tests. It fixes a bug that caused incorrect paths when joining empty strings or strings with separators, and adds more test cases for different operating systems and scenarios.

🤖 Generated by Copilot at a00c73b

• Fix a bug in os.join_path that would produce incorrect paths with empty strings or leading separators (link)
• Add more test cases for os.join_path in os_test.v, covering empty and separator scenarios and using conditional compilation for different OSes (link)