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

(cherry picked from commit 4eebc46930)

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,