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

Author: SergeyBiryukov

src/wp-includes/link-template.php

@@ -2649,7 +2649,8 @@ function get_previous_posts_page_link() {
  * @return string|null The previous posts page link if `$display = false`.
  */
 function previous_posts( $display = true ) {
-	$output = esc_url( get_previous_posts_page_link() );
+	$link   = get_previous_posts_page_link();
+	$output = $link ? esc_url( $link ) : '';
 
 	if ( $display ) {
 		echo $output;

tests/phpunit/tests/link/previousPosts.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Tests the `previous_posts()` function.
+ *
+ * @since 7.0.0
+ *
+ * @group link
+ *
+ * @covers ::previous_posts
+ */
+class Tests_Link_PreviousPosts extends WP_UnitTestCase {
+
+	/**
+	 * The absence of a deprecation notice on PHP 8.1+ also shows that the issue is resolved.
+	 *
+	 * @ticket 64864
+	 */
+	public function test_should_return_empty_string_on_singular() {
+		$post_id = self::factory()->post->create();
+		$this->go_to( get_permalink( $post_id ) );
+
+		$this->assertSame( '', previous_posts( false ) );
+	}
+}