Fix the broken image hyperlink content element

Motivation:
* Instead of an image hyperlink a normal text link is shown

Due to the [change](https://github.com/contao/core/commit/0b1621f49f5702ddfa16a78b6c938c61bd59b3aa#diff-2) of the SQL `singleSRC` field type from `char` to `binary` the [check for a numerical field value](https://github.com/contao/core/blob/develop/system/modules/core/elements/ContentHyperlink.php#L62) fails now which in turns means that e.g. the front end template object for `ce_hyperlink_image` cannot be instantiated.

Credit goes to @peter.fl who found and reported the issue.