[com_fields] Fix notices in contact view

[Bakual]

Pull Request for Issue #12728

Summary of Changes

• render_class is a param, not an object property.
• The $fields array doesn't always start with the key 0. This PR uses reset to get the first item.

Testing Instructions

• Create multiple custom fields for contacts and set the "edit value" permissions so it is allowed by public. Set the fields to different positions (unter tab options -> automatic display)
• Edit a contact to add some value to those fields.
• View that contact in frontend.

Before PR you get multiple notices after PR they are gone.

Documentation Changes Required

None

[RickR2H]

I have tested this item ✅ successfully on 188cfeb

Notices where gone after patch install

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[AnnemiekStoel]

I have tested this item ✅ successfully on 188cfeb

Test succesful

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[zero-24]

RTC

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