better matching of empty server banners

Backport of b1b30b3

Author: drwetter

testssl.sh

@@ -2013,7 +2013,7 @@ run_server_banner() {
      grep -ai '^Server' $HEADERFILE >$TMPFILE
      if [[ $? -eq 0 ]]; then
           serverbanner=$(sed -e 's/^Server: //' -e 's/^server: //' $TMPFILE)
-          if [[ x"$serverbanner" == "x\n" ]] || [[ x"$serverbanner" == "x\n\r" ]] || [[ -z "$serverbanner" ]]; then
+          if [[ "$serverbanner" == $'\n' ]] || [[ "$serverbanner" == $'\r' ]] || [[ "$serverbanner" == $'\n\r' ]] || [[ -z "$serverbanner" ]]; then
                outln "banner exists but empty string"
                fileout "serverbanner" "INFO" "Server banner exists but empty string"
           else