hugolib: Fix handling of zero-length files

This was a regression in Hugo 0.20. This commit makes sure that zeron-length files are not rendered to file.

Fixes #3355

hugolib/site_render.go

@@ -340,6 +340,10 @@ func (s *Site) renderRobotsTXT() error {
 		return nil
 	}
 
+	if outBuffer.Len() == 0 {
+		return nil
+	}
+
 	return s.publish("robots.txt", outBuffer)
 }