MDL-67114 core: php74 fix. Fix use of scalar as array in core

There are various places where it's not guaranteed that the
variable being used is array, and instead, can be null, bool, int...

We need to check that because php74 warns about it.

Where possible we have used the coalesce operator as
replacement for isset() ternary operations.

mod/wiki/parser/parser.php

@@ -47,7 +47,7 @@ class wiki_parser_proxy {
             	return $content;
             }
             else {
-            	return $content[1];
+            	return is_array($content) ? $content[1] : null;
         	}
         }
         else {