fix(startup): "nvim -l foo.lua" may not set arg0 #24161

Problem:
Using "nvim -l args.lua" without passing extra script args, does not set `_G.arg[0]`.

Steps to reproduce:
```
cat > args.lua<<EOF
vim.print(_G.arg, '')
vim.print(vim.v.argv, '')
EOF

nvim --clean -l args.lua
```

Solution:
Fix condition in command_line_scan.

test/functional/core/startup_spec.lua

@@ -144,6 +144,18 @@ describe('startup', function()
     end)
 
     it('sets _G.arg', function()
+      -- nvim -l foo.lua
+      assert_l_out([[
+          bufs:
+          nvim args: 3
+          lua args: {
+            [0] = "test/functional/fixtures/startup.lua"
+          }]],
+        {},
+        {}
+      )
+      eq(0, eval('v:shell_error'))
+
       -- nvim -l foo.lua [args]
       assert_l_out([[
           bufs: