fix(vim.system): env=nil passes env=nil to uv.spawn

731e616a79 made it so passing `{env = nil, clear_env = true }` would
pass `{env = {}}` to `vim.uv.spawn`.

However this is not what `clear_env` is (arguably) supposed to do.
If `env=nil` then that implies the uses wants `vim.uv.spawn()` to use
the default environment. Adding `clear_env = true` simply prevents
`NVIM` (the base environment) from being added.

Fixes #34730

runtime/doc/lua.txt

@@ -1785,7 +1785,9 @@ vim.system({cmd}, {opts}, {on_exit})                            *vim.system()*
                      the new process. Inherits the current environment with
                      `NVIM` set to |v:servername|.
                    • clear_env: (boolean) `env` defines the job environment
-                     exactly, instead of merging current environment.
+                     exactly, instead of merging current environment. Note: if
+                     `env` is `nil`, the current environment is used but
+                     without `NVIM` set.
                    • stdin: (string|string[]|boolean) If `true`, then a pipe
                      to stdin is opened and can be written to via the
                      `write()` method to SystemObj. If string or string[] then

runtime/lua/vim/_editor.lua

@@ -106,7 +106,7 @@ local utfs = {
 ---   - env: table<string,string> Set environment variables for the new process. Inherits the
 ---     current environment with `NVIM` set to |v:servername|.
 ---   - clear_env: (boolean) `env` defines the job environment exactly, instead of merging current
----     environment.
+---     environment. Note: if `env` is `nil`, the current environment is used but without `NVIM` set.
 ---   - stdin: (string|string[]|boolean) If `true`, then a pipe to stdin is opened and can be written
 ---     to via the `write()` method to SystemObj. If string or string[] then will be written to stdin
 ---     and closed. Defaults to `false`.

runtime/lua/vim/_system.lua

@@ -208,13 +208,18 @@ end
 --- @param clear_env? boolean
 --- @return string[]?
 local function setup_env(env, clear_env)
+  if not env and clear_env then
+    return
+  end
+
+  env = env or {}
   if not clear_env then
     --- @type table<string,string|number>
-    env = vim.tbl_extend('force', base_env(), env or {})
+    env = vim.tbl_extend('force', base_env(), env)
   end
 
   local renv = {} --- @type string[]
-  for k, v in pairs(env or {}) do
+  for k, v in pairs(env) do
     renv[#renv + 1] = string.format('%s=%s', k, tostring(v))
   end
 

test/functional/lua/system_spec.lua

@@ -93,6 +93,27 @@ describe('vim.system', function()
         )
       end)
 
+      it('can set environment with clear_env = true and env = nil', function()
+        exec_lua(function()
+          vim.env.TEST = 'TESTVAL'
+        end)
+
+        -- Not passing env with clear_env should not clear the environment
+        eq(
+          'TESTVAL',
+          system({ n.testprg('printenv-test'), 'TEST' }, { clear_env = true, text = true }).stdout
+        )
+
+        -- Passing env = {} with clear_env should clear the environment
+        eq(
+          '',
+          system(
+            { n.testprg('printenv-test'), 'TEST' },
+            { env = {}, clear_env = true, text = true }
+          ).stdout
+        )
+      end)
+
       it('supports timeout', function()
         eq({
           code = 124,