fix: global import from an array value

[mRoca]

Type: bugfix
Breaking change: no

This PR fixes a PHP issue when running the following code with the MissingImport's ignore-global option enabled:

$classes = ['\stdClass'];
$b = new $classes[0];

The error:

ValueError: strpos(): Argument #3 ($offset) must be contained in argument #1 ($haystack)

src/main/php/PHPMD/Rule/CleanCode/MissingImport.php:92

The reason:

In this case, the current ASTNode will be a PDepend\Source\AST\ASTArrayIndexExpression, and $classNode->getImage() will return an empty string

[kylekatarnls]

Thanks 🙏

[AJenbo]

Thanks for adding a test for this