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The file robots.txt won't help your rankings. Having such a file on your server will prevent getting a 404 error every time it is asked.
If you don't have one, and use custom 404 page(s), make sure you're returning the 404 header so that your custom 404 page won't be interpreted as a robots.txt with a lot of "syntax errors".
If you use the Apache directive "ErrorDocument" in a .htaccess file, avoid using full URLs for the custom 404 page (no http prefix), i.e.:
ErrorDocument 404 /your-good-404-page.html
and not:
ErrorDocument 404 [domain.tld...]
// previous line is wrong as it doesn't return the correct header!
Dan