[DowngradePhp80] Add DowngradeSubstrFalsyRector

Author: samsonasik

rules/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector.php

@@ -52,7 +52,7 @@ public function refactor(Node $node): ?Node
         }
 
         if ($node instanceof Ternary) {
-            if ($node->if instanceof Expr) {
+            if (! $node->if instanceof Expr) {
                 $node->cond->setAttribute(self::IS_UNCASTABLE, true);
             }