fix(lua): vim.split may trim inner empty items

Problem:
`vim.split('a:::', ':', {trimempty=true})` trims inner empty items.
Regression from 9c49c10470

Solution:
Set `empty_start=false` when first non-empty item is found.
close #23212

runtime/lua/vim/shared.lua

@@ -138,7 +138,9 @@ function vim.gsplit(s, sep, opts)
     local seg = _pass(s:find(sep, start, plain))
 
     -- Trim empty segments from start/end.
-    if trimempty and seg == '' then
+    if seg ~= '' then
+      empty_start = false
+    elseif trimempty then
       while not done and seg == '' do
         empty_segs = empty_segs + 1
         seg = _pass(s:find(sep, start, plain))