Fix PHP warnings when building an incomplete CASE expression

Issue can be reproduced with the following code snippet:
```php
new \PhpMyAdmin\SqlParser\Parser('SELECT a CASE');
// PHP Warning:  count(): Parameter must be an array or an object that implements Countable in src/Components/CaseExpression.php on line 296
// PHP Warning:  count(): Parameter must be an array or an object that implements Countable in src/Components/CaseExpression.php on line 297
```

1 files changed, 3 insertions, 3 deletions

diff --git a/src/Components/CaseExpression.php b/src/Components/CaseExpression.php
index a79b52e..309f0ff 100644
--- a/src/Components/CaseExpression.php
+++ b/src/Components/CaseExpression.php
@@ -33,21 +33,21 @@ class CaseExpression extends Component
      *
      * @var array
      */
-    public $conditions;
+    public $conditions = [];
 
     /**
      * The results matching with the WHEN clauses.
      *
      * @var array
      */
-    public $results;
+    public $results = [];
 
     /**
      * The values to be compared against.
      *
      * @var array
      */
-    public $compare_values;
+    public $compare_values = [];
 
     /**
      * The result in ELSE section of expr.