[4.x] Remove double-render of fields in Form tag

[martyf]

In the Forms tag, when create is called, getSections and getFields are called to get the sections and fields respectively.

However, getSections already calls getFields. This results in getFields being called twice - meaning fields are being rendered twice.

This PR changes the behaviour of create and runs getSections first, as it currently does, and then checks to see if sections were returned - if so, it then extracts the fields from the sections (rather than calling getFields again). If there are no sections (i.e. think historical pre-section forms) then getFields is explicitly called.

[jasonvarga]

Pretty sure that getSections is going to populate the fields regardless of what your blueprint looks like since it all gets normalized.

I think we can just assume sections will be good to go and you can pick the fields from there, rather than doing the condition.

I'll double check though.