fix(vim.fs): joinpath() should ignore empty items #38077

Problem:
vim.fs.joinpath treats empty string as a path segment
(it adds a path separator for each empty item):

    print(vim.fs.joinpath('', 'after/lsp', '')) -- '/after/lsp/'
    print(vim.fs.joinpath('', '')) -- '/'

Especially problematic if the empty segment is the first segment, as
that converts the path to an absolute path.

Solution:
Ignore empty (length of 0) path segments.

Benchmark:

    local function test(func)
      local t = vim.uv.hrtime()
      for _ = 1, 100000, 1 do
        func('', 'this/is', 'a/very/long/path', '', 'it', 'really', 'is')
      end
      print(math.floor((vim.uv.hrtime() - t) / 1e6), 'ms')
    end

- with Iter():filter() --> 370 ms
- building new segments table --> 208 ms
- with vim.tbl_filter --> 232 ms
- Instead of gsub split on `/` in all parts --> 1870 ms

test/functional/lua/fs_spec.lua

@@ -470,6 +470,12 @@ describe('vim.fs', function()
         eq('foo/bar/baz/zub/', vim.fs.joinpath([[foo]], [[//bar////baz]], [[zub/]]))
       end
     end)
+    it('handles empty segments', function()
+      eq('foo/bar', vim.fs.joinpath('', 'foo', '', 'bar', ''))
+      eq('foo/bar', vim.fs.joinpath('', '', 'foo', 'bar', '', ''))
+      eq('', vim.fs.joinpath(''))
+      eq('', vim.fs.joinpath('', '', '', ''))
+    end)
   end)
 
   describe('normalize()', function()