Meta thumbnail URL errors when page slug is called `assets`

[lukasbestle]

E.g. https://getkirby.com/docs/reference/system/urls/assets (error from deprecation only visible with debug mode):

Whoops\Exception\ErrorException thrown with message "file_exists(): Passing null to parameter #1 ($filename) of type string is deprecated"

Stacktrace:
#28 Whoops\Exception\ErrorException in /var/www/site/plugins/cdn/src/Optimizer.php:55
#27 file_exists in /var/www/site/plugins/cdn/src/Optimizer.php:55
#26 Kirby\Cdn\Optimizer:cachebust in /var/www/site/plugins/cdn/index.php:64
#25 Kirby\Filesystem\F:{closure} in /var/www/kirby/src/Cms/Url.php:82
#24 Kirby\Cms\Url:to in /var/www/kirby/config/helpers.php:627
#23 url in /var/www/site/plugins/meta/src/PageMeta.php:154
#22 Kirby\Meta\PageMeta:social in /var/www/site/snippets/layouts/head.php:37
#21 include in /var/www/kirby/src/Filesystem/F.php:434
#20 Kirby\Filesystem\F:loadIsolated in /var/www/kirby/src/Filesystem/F.php:374
#19 Kirby\Filesystem\F:Kirby\Filesystem\{closure} in /var/www/kirby/src/Filesystem/F.php:380
#18 Kirby\Filesystem\F:load in /var/www/kirby/src/Toolkit/Tpl.php:35
#17 Kirby\Toolkit\Tpl:load in /var/www/kirby/src/Template/Snippet.php:174
#16 Kirby\Template\Snippet:factory in /var/www/kirby/config/components.php:312
#15 Kirby\Cms\Core:{closure} in /var/www/kirby/src/Cms/App.php:1602
#14 Kirby\Cms\App:snippet in /var/www/kirby/config/helpers.php:533
#13 snippet in /var/www/site/layouts/reference.php:4
#12 include in /var/www/kirby/src/Filesystem/F.php:434
#11 Kirby\Filesystem\F:loadIsolated in /var/www/kirby/src/Filesystem/F.php:374
#10 Kirby\Filesystem\F:Kirby\Filesystem\{closure} in /var/www/kirby/src/Filesystem/F.php:380
#9 Kirby\Filesystem\F:load in /var/www/kirby/src/Toolkit/Tpl.php:35
#8 Kirby\Toolkit\Tpl:load in /var/www/kirby/src/Template/Snippet.php:250
#7 Kirby\Template\Snippet:render in /var/www/kirby/src/Template/Template.php:184
#6 Kirby\Template\Template:render in /var/www/kirby/src/Cms/Page.php:988
#5 Kirby\Cms\Page:Kirby\Cms\{closure} in /var/www/kirby/src/Content/VersionId.php:118
#4 Kirby\Content\VersionId:render in /var/www/kirby/src/Cms/Page.php:966
#3 Kirby\Cms\Page:render in /var/www/kirby/src/Cms/App.php:803
#2 Kirby\Cms\App:io in /var/www/kirby/src/Cms/App.php:1218
#1 Kirby\Cms\App:render in /var/www/index.php:7
#0 require in /Applications/Herd.app/Contents/Resources/valet/server.php:167