Handle case where paid plugin update is put into no_update array

To avoid warning:

PHP Notice:  Undefined property: stdClass::$requires in
/usr/local/etc/wp-cli/packages/vendor/wp-cli/extension-command/src/Plugin_Command.php
on line 875

Author: mrsdizzie

features/plugin.feature

@@ -880,6 +880,51 @@ Feature: Manage WordPress plugins
       Warning: example: This update requires WordPress version 100
       """
 
+  @require-wp-4.0
+  Scenario: Show plugin update as unavailable if it has a new version but no update package provided by author
+    Given a WP install
+    And a wp-content/plugins/example/example.php file:
+      """
+      <?php
+        /**
+        * Plugin Name: Example Plugin
+        * Version: 1.0.0
+        * Requires at least: 3.7
+        * Tested up to: 6.7
+      """
+    And that HTTP requests to https://api.wordpress.org/plugins/update-check/1.1/ will respond with:
+      """
+      HTTP/1.1 200 OK
+
+      {
+        "plugins": [],
+        "translations": [],
+        "no_update": {
+          "example/example.php": {
+            "id": "w.org/plugins/example",
+            "slug": "example",
+            "plugin": "example/example.php",
+            "new_version": "2.0.0",
+
+            "requires_plugins": [],
+            "compatibility": []
+          }
+        }
+      }
+      """
+
+    When I run `wp plugin list`
+    Then STDOUT should be a table containing rows:
+      | name            | status   | update       | version  | update_version   | auto_update | requires   | requires_php   |
+      | example         | inactive | unavailable  | 1.0.0    | 2.0.0            | off         |            |                |
+
+    When I try `wp plugin update example`
+    Then STDERR should contain:
+      """
+      Warning: example: Update file not provided. Contact author for more details
+      """
+
+
   @require-wp-4.0
   Scenario: Show plugin update as unavailable if it doesn't meet PHP requirements
     Given a WP install

src/Plugin_Command.php

@@ -864,15 +864,22 @@ protected function get_item_list() {
 					$items[ $file ]['requires_php'] = isset( $plugin_update_info->requires_php ) ? $plugin_update_info->requires_php : null;
 				}
 
-				// If there is a plugin in no_update with a newer version than the local copy, it is because the plugins update api
-				// has already filtered it because the local WordPress version is too low
+				// If there is a plugin in no_update with a newer version than the local copy, it is either because:
+				// A: the plugins update API has already filtered it because the local WordPress version is too low
+				// B: It is possibly a paid plugin that has an update which the user does not qualify for
 				if ( null !== $plugin_update_info && version_compare( $details['Version'], $plugin_update_info->new_version, '<' ) ) {
 					$items[ $file ]['update']         = 'unavailable';
 					$items[ $file ]['update_version'] = $plugin_update_info->new_version;
 					$items[ $file ]['requires']       = isset( $plugin_update_info->requires ) ? $plugin_update_info->requires : null;
 					$items[ $file ]['requires_php']   = isset( $plugin_update_info->requires_php ) ? $plugin_update_info->requires_php : null;
 
-					$reason = "This update requires WordPress version $plugin_update_info->requires, but the version installed is $wp_version.";
+					if ( isset( $plugin_update_info->requires ) && version_compare( $wp_version, $requires, '>=' ) ) {
+						$reason = "This update requires WordPress version $plugin_update_info->requires, but the version installed is $wp_version.";
+					} elseif ( ! isset( $update_info['package'] ) ) {
+						$reason = 'Update file not provided. Contact author for more details';
+					} else {
+						$reason = 'Update not available';
+					}
 
 					$items[ $file ]['update_unavailable_reason'] = $reason;