startup: go to buffer 2 if stdin is empty

If stdin is not a TTY we read it into buffer 1, as text. But if the
stdin pipe is empty, Nvim was most likely invoked for some other reason.
DWIM: select buffer 2 (if it exists). Example:

    echo file1 | xargs nvim

closes #8560
closes #8561
ref https://github.com/equalsraf/neovim-qt/issues/417

test/functional/core/startup_spec.lua

@@ -122,6 +122,18 @@ describe('startup', function()
                     { 'ohyeah', '' }))
   end)
 
+  it('goes to buffer 2 if stdin is empty #8561', function()
+    eq('\r\n  1u#    "[No Name]"                    line 1\r\n  2 %a   "file1"                        line 0\r\n  3      "file2"                        line 0',
+       funcs.system({nvim_prog, '-n', '-u', 'NONE', '-i', 'NONE', '--headless',
+                     '+ls!',
+                     '+qall!',
+                     '-',
+                     'file1',
+                     'file2',
+                    },
+                    { '' }))
+  end)
+
   it('-e/-E interactive #7679', function()
     clear('-e')
     local screen = Screen.new(25, 3)