Change check of the array key "value" from isset to array_key_exists

The problem occured if inlcudeBlankOption is set to true in the dca and the sql field type is varbinary cause "https://github.com/contao/core/blob/master/system/modules/core/library/Contao/Widget.php#L1377" returns null to the value key and isset failes if the value of an array key is null