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
```

Author: LeSuisse

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.

tests/Components/CaseExpressionTest.php

@@ -118,4 +118,10 @@ public function testParseBuild8()
             'CASE WHEN 1=1 THEN "Some" WHEN 1=2 THEN "SomeOther" END AS `foo`'
         );
     }
+
+    public function testBuildWithIncompleteCaseExpression()
+    {
+        $incomplete_case_expression_component = new CaseExpression();
+        $this->assertEquals('CASE END', CaseExpression::build($incomplete_case_expression_component));
+    }
 }