Fixed issue where View Assigned Assets would crash if there are no custom fields for any assigned models

[snipe]

If none of a user's assigned assets have any custom fields, this will cause the View Assigned Assets page to crash. This fixes that regression.

[what-the-diff]

• Added a new variable $field_array
• Changed the foreach loop to check if asset->model has fieldset before checking for fields in it
• Moved the code that adds custom fields into an array outside of both loops and added another condition to make sure they're allowed to see the value of this custom field

[uberbrady]

LGTM!