Add is_object check in Logger

[fabian]

Fix for PHP 7.2 compatibility, get_class doesn't like null values anymore.

[fabian]

{
    "error": {
        "type": "Whoops\\Exception\\ErrorException",
        "message": "get_class() expects parameter 1 to be object, null given",
        "file": "/var/www/html/packages/migration_tool/src/PortlandLabs/Concrete5/MigrationTool/Publisher/Logger/Logger.php",
        "line": 120,
        "trace": [
            {
                "file": "/var/www/html/packages/migration_tool/src/PortlandLabs/Concrete5/MigrationTool/Publisher/Logger/Logger.php",
                "line": 120,
                "function": null,
                "class": "Whoops\\Exception\\ErrorException",
                "args": [
                    "get_class() expects parameter 1 to be object, null given"
                ]
            }
        ]
    },
    "errors": [
        "get_class() expects parameter 1 to be object, null given"
    ]
}