Code Modernization: Fix "passing null to non-nullable" deprecation from `previous_posts()`.

The `esc_url()` function expects to a string for `$url` parameter. There is no input validation within that function. The function contains a `ltrim()` which also expects a string. Passing `null` to this parameter results in `Deprecated: ltrim(): Passing null to parameter #1 ($string) of type string is deprecated` notice on PHP 8.1+.

Tracing the stack back, a `null` is being passed to it within `previous_posts()` when `get_previous_posts_page_link()` returns `null` (it can return a string or `null`).

On PHP 7.0 to PHP 8.x, an empty string is returned from `esc_url()` when `null` is passed to it. The change in this changeset avoids the deprecation notice by not invoking `esc_url()` when `get_previous_posts_page_link()` returns `null` and instead sets the `$output` to an empty string, thus maintaining the same behavior as before (minus the deprecation notice).

Adds a test to validate an empty string is returned and the absence of the deprecation (when running on PHP 8.1+).

Follow-up to [9632], [11383], [56740].

Props dd32, alexodiy.
Fixes #64864.

git-svn-id: https://develop.svn.wordpress.org/trunk@62034 602fd350-edb4-49c9-b593-d223f7449a82