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, 6 insertions, 0 deletions

diff --git a/tests/Components/CaseExpressionTest.php b/tests/Components/CaseExpressionTest.php
index cb9290c..2ea8f8b 100644
--- a/tests/Components/CaseExpressionTest.php
+++ b/tests/Components/CaseExpressionTest.php
@@ -118,4 +118,10 @@ class CaseExpressionTest extends TestCase
             '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));
+    }
 }