apply_filters_deprecated() and do_action_deprecated() should only trigger a notice when a callback is hooked

[johnbillion]

The inline docs for apply_filters_deprecated() includes the following example code:

// Old filter.
return apply_filters( 'wpdocs_filter', $value, $extra_arg );

// Deprecated.
return apply_filters_deprecated( 'wpdocs_filter', array( $value, $extra_arg ), '4.9', 'wpdocs_new_filter' );

The problem with this code is that the deprecated notice is triggered regardless of whether a callback is hooked into the deprecated filter or not. The deprecated notice should only be triggered if a callback is hooked into the filter.

Two options to fix this. Either:

1. Update the example code to include a check for has_filter()/has_action(), or
2. Add a check for has_filter()/has_action() inside apply_filters_deprecated()/do_action_deprecated() so the notice is only triggered when a callback is hooked.

[SergeyBiryukov]

Add a check for has_filter()/has_action() inside apply_filters_deprecated()/do_action_deprecated() so the notice is only triggered when a callback is hooked.

For reference, re-reading [37861], it looks like that's already the case.