Plugins: Consistently use an empty string as the default value for...

Plugins: Consistently use an empty string as the default value for `$replacement` and `$message` parameters in:

* `_deprecated_function()`
* `_deprecated_constructor()`
* `_deprecated_file()`
* `_deprecated_argument()`
* `_deprecated_hook()`
* `apply_filters_deprecated()`
* `do_action_deprecated()`

This matches the documented type of `string` for these parameters and removes unnecessarily strict `! is_null()` checks.

Follow-up to [46792].

Props jignesh.nakrani, renathoc, SergeyBiryukov.
Fixes #49698.
Built from https://develop.svn.wordpress.org/trunk@48327


git-svn-id: https://core.svn.wordpress.org/trunk@48096 1a063a9b-81f0-0310-95a4-ce76da25c4cd