fix: tostring(vim.version()) fails if build is NIL #24097

Problem:
Since #23925, Version.build may be vim.NIL, which causes tostring() to fail:
    E5108: Error executing lua E5114: Error while converting print argument #1: …/version.lua:129:
    attempt to concatenate field 'build' (a userdata value)
    stack traceback:
            [C]: in function 'print'
            [string ":lua"]:1: in main chunk

Solution:
Handle vim.NIL in Version:__tostring().

runtime/lua/vim/version.lua

@@ -125,7 +125,7 @@ function Version:__tostring()
   if self.prerelease then
     ret = ret .. '-' .. self.prerelease
   end
-  if self.build then
+  if self.build and self.build ~= vim.NIL then
     ret = ret .. '+' .. self.build
   end
   return ret