fix(health): bad format() call #34906

Problem:
Bad format() call on PUC Lua

    Error: Failed to run healthcheck for "vim.health" plugin. Exception:
    runtime/lua/vim/health/health.lua:89: bad argument #1 to 'format' (string expected, got nil)

Solution:
Avoid passing nil.

(cherry picked from commit 2422fbdd5f)

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/lua/vim/health/health.lua

@@ -87,9 +87,9 @@ local function check_config()
     health.error(
       'Locale does not support UTF-8. Unicode characters may not display correctly.'
         .. ('\n$LANG=%s $LC_ALL=%s $LC_CTYPE=%s'):format(
-          vim.env.LANG,
-          vim.env.LC_ALL,
-          vim.env.LC_CTYPE
+          vim.env.LANG or '',
+          vim.env.LC_ALL or '',
+          vim.env.LC_CTYPE or ''
         ),
       {
         'If using tmux, try the -u option.',