`NullConditionalExpression` should also be used in a fluent style

Now the only way to construct the following expression
```csharp
input?.ToList()
```
is
```csharp
new NullConditionalExpression(input).ToList()
```
which is not following our style of fluent expression usages, we should update to have:
```csharp
input.NullConditional().ToList()
```