MDL-87789 libraries: patch lti1p3 for certification support

https://github.com/packbackbooks/lti-1-3-php-library/issues/169 blocks
us from using the new build.1edtech certification tooling. This issue
commits that fix as a library customisation/hack for stables, so the
suite can be used for annual/bi-annual re-certification.

lib/lti1p3/readme_moodle.txt

@@ -2,6 +2,7 @@ LTI 1.3 Tool Library import instructions
 
 This library is a patched for use in Moodle - it requires the following changes be applied on top of the packback upstream base:
 1. Removal of phpseclib dependency (replaces a single call with openssl equivalent)
+2. The fix included in MDL-87789, ensuring optional lineitem properties are omitted if not set.
 
 To upgrade to a new version of this library:
 1. Clone the latest version of the upstream library from github:

lib/lti1p3/src/DeepLinkResources/Resource.php

@@ -50,7 +50,7 @@ class Resource
         if (isset($this->line_item)) {
             $resource['lineItem'] = [
                 'scoreMaximum' => $this->line_item->getScoreMaximum(),
-                'label' => $this->line_item->getLabel(),
+                ...(!is_null($this->line_item->getLabel()) ? ['label' => $this->line_item->getLabel()] : []),
             ];
         }