Set AMP_QUERY_VAR fallback when AMP is inactive
Set AMP_QUERY_VAR fallback when AMP is inactive

If AMP is deactivated via the `amp_is_enabled` filter, the
`AMP_QUERY_VAR` ends up undefined. This causes warnings when a helper
function like `is_amp_endpoint()` is called.

Setting a fallback value prevents this.

ampproject/amp-toolbox-php,ampproject/amp-toolbox-php