Plugins: Correct default value of `$replacement` parameter in...

Plugins: Correct default value of `$replacement` parameter in `do_action_deprecated()` and `apply_filters_deprecated()`.

This addresses an inconsistency with `_deprecated_hook()`, which uses `is_null()` to check if `$replacement` was provided, however the previous default value was `false`.

Props shaampk1, felipeelia.
Fixes #48817.
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git-svn-id: https://core.svn.wordpress.org/trunk@46592 1a063a9b-81f0-0310-95a4-ce76da25c4cd