@Nullable
    private Object convert(@Nullable String value) {
        if (value == null) {
            return defaultOrException();
        }
        if (value.isEmpty()) {
            if (String.class.isAssignableFrom(elementType)) {
                return value;
            }
            if (elementType.isPrimitive()) {
                throw new IllegalArgumentException("Cannot set null to primitive type parameter: " +
                                                   httpElementName);
            }
            return null;
        }
        return convert(value, elementType, enumConverter);
    }
Returns an attribute resolver which retrieves a value specified by {@code attrKeys}
from the {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java