fix GetVersionDetails error

[TylerLeonhardt]

We were seeing this exception every time on macOS.

2018-02-23 20:23:14 [WARNING] - Method "GetVersionDetails" at line 153 of C:\projects\powershelleditorservices\src\PowerShellEditorServices\Session\PowerShellVersionDetails.cs

    Failed to look up PowerShell version, defaulting to version 5.
    
    System.NullReferenceException: Object reference not set to an instance of an object.
       at Microsoft.PowerShell.EditorServices.PowerShellContext.<>c__56`1.<ExecuteScriptAndGetItem>b__56_0(PSObject pso)
       at System.Linq.Enumerable.SelectIListIterator`2.MoveNext()
       at System.Linq.Enumerable.<OfTypeIterator>d__32`1.MoveNext()
       at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)
       at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
       at Microsoft.PowerShell.EditorServices.PowerShellContext.ExecuteScriptAndGetItem[TResult](String scriptToExecute, Runspace runspace, TResult defaultValue)
       at Microsoft.PowerShell.EditorServices.Session.PowerShellVersionDetails.GetVersionDetails(Runspace runspace, ILogger logger)

It's because $env:PROCESSOR_ARCHITECTURE does not exist on macOS and the result of this line returned a collection that had one value... null - rather than returning just null.

This fix returns just null if the contents of the execution contains just an null item.

Resolves PowerShell/vscode-powershell#1211
Resolves #636

[rkeithhill]

What if we changed that line to this:

var arch = (IntPtr.Size == 8) ? "AMD64" : "x86";

I know, on an 32-bit ARM system "x86" isn't quite right but at least it conveys it is a 32-bit system ... not that we have PSES running on 32-bit ARM - do we?

[rkeithhill]

LGTM but you might want to have David look at this one. I think this method might be used in a lot of places.

[TylerLeonhardt]

I thought about var arch = (IntPtr.Size == 8) ? "AMD64" : "x86"; but I was worried about changing behavior. At least with my PR, nothing changes (that code just doesn't throw an exception that gets logged).

That said, if your line can give some better perf in some way, I'm all for it! I'll wait for @daviwil to chime in.

[SeeminglyScience]

LGTM, though we may want to open an issue to replace any uses of the pipeline API with nested PowerShell instances. Looks like that API is on the chopping block

[TylerLeonhardt]

@SeeminglyScience good point. Let me open an issue on that.

[TylerLeonhardt]

Alright it's been a month. I'm just going to merge this in.