Make JSON more robust

In cases where a finding was empty (error condition), the JSON output
wasn't valid because the finding wasn't printed to file.

This commit makes sure that always a finding is printed,
also if it is empty.

FIX #1112

testssl.sh

@@ -753,7 +753,7 @@ fileout_json_print_parameter() {
      "$do_json" && \
          spaces="              " || \
          spaces="                                "
-    if [[ ! -z "$value" ]]; then
+     if [[ -n "$value" ]] || [[ "$parameter" == finding ]]; then
           printf "%s%s%s%s" "$spaces" "\"$parameter\"" "$filler" ": \"$value\"" >> "$JSONFILE"
           "$not_last" && printf ",\n" >> "$JSONFILE"
      fi