Fix League\CommonMark\Converter::convertToHtml() must be of the type …

…string, null given exception

Laravel v6.10.0 switched to "league/commonmark instead of erusev/parsedown for mail markdown"

league/commonmark doesn't accept null to the parse method. So this small update pass in an empty string if it is null.

src/resources/views/crud/columns/markdown.blade.php

@@ -1 +1 @@
-{!! Illuminate\Mail\Markdown::parse($entry->{$column['name']}) !!}
+{!! Illuminate\Mail\Markdown::parse($entry->{$column['name']} ?? '') !!}