MDL-80677 core: deprecation::from should always return

lib/classes/deprecation.php

@@ -83,6 +83,9 @@ class deprecation {
             // The reference is an array, but it's not an object or a class that currently exists.
             return null;
         }
+
+        // The reference is none of the above.
+        return null;
     }
 
     /**

lib/tests/deprecation_test.php

@@ -285,6 +285,9 @@ class deprecation_test extends \advanced_testcase {
             // Deprecated global function.
             ['core\fixtures\deprecated_function', true],
             ['core\fixtures\not_deprecated_function', false],
+
+            // Empty array.
+            [[], false],
         ];
     }