Fixed 500 if format is not passed to the create custom field endpoint payload

[snipe]

Previously, if you did not include format in your new custom field API call, the call would fail with a 500 error that looks like this:

"message": "Undefined index: format",
    "exception": "ErrorException",
    "file": "/Users/snipe/Sites/snipe-it/snipe-it/app/Http/Controllers/Api/CustomFieldsController.php",
    "line": 99,

This fix corrects that behavior and checks to see that the format index actually exists in the $data payload.

[what-the-diff]

• Added a check to see if the format key exists in $data
• If it does, then we will continue with our regex logic