[.NET Templating] Resolve commas added in wrong place

[anna-dingler]

Related Issue

Fixes #6680

Description

Commas were sometimes being added unnecessarily which resulted in invalid JSON. I updated TemplateVisitor to only add commas if there is an object prior and another object is about to be added.

Sample Card

Card:

{
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "type": "AdaptiveCard",
    "version": "1.3",
    "body": [
      {
        "type": "ColumnSet",
        "$data": "${foo}",
        "$when": "${$index==0}"
      },
      {
        "type": "Container",
        "$data": "${foo}",
        "$when": "${$index>0}"
      }
    ]
}

Data:

{
	"foo": [{}, {}]
}

How Verified

Verified manually and with the added unit tests.

Microsoft Reviewers: Open in CodeFlow

[beervoley]