Check image property is set in Object\Image

- Property was either an object or null, strict comparison with "false" was inaccurate
- Address https://github.com/friendica/friendica/issues/12486#issuecomment-1374888800

src/Object/Image.php

@@ -196,7 +196,7 @@ class Image
 	public function isValid(): bool
 	{
 		if ($this->isImagick()) {
-			return ($this->image !== false);
+			return !empty($this->image);
 		}
 		return $this->valid;
 	}