@Nullable
        String getFirstValueOrNull(String name) {
            requireNonNull(name, "name");
            final String value = params.get(name);
            return Strings.emptyToNull(value);
        }
Returns the first value of the parameter with the specified {@code name}. If it does not exist,
{@code null} is returned.
saml/src/main/java/com/linecorp/armeria/server/saml/SamlService.java