startup: fix -E/-Es without -u NONE

Before this change, -E/-Es without `-u NONE` reads stdin as Ex commands.
It should always read stdin as text (into buffer 1), like this:

    echo foo | nvim -Es +'%p'
    foo
    echo foo | nvim -Es -u NORC +'%p'
    foo

src/nvim/main.c

@@ -301,9 +301,11 @@ int main(int argc, char **argv)
   // Read ex-commands if invoked with "-es".
   //
   bool reading_tty = !headless_mode
+                     && !silent_mode
                      && (params.input_isatty || params.output_isatty
                          || params.err_isatty);
-  bool reading_excmds = !params.input_isatty && silent_mode
+  bool reading_excmds = !params.input_isatty
+                        && silent_mode
                         && exmode_active == EXMODE_NORMAL;
   if (reading_tty || reading_excmds) {
     // One of the startup commands (arguments, sourced scripts or plugins) may