Why does Powershell silently convert a string array with one item to a string

Evidently, PowerShell implicitly 'unboxes' a single-item array to a single object,

And zero item results to $null.

How can I prevent this from happening?

You can't.

How do you deal with this?

Use the array constructor (@(...)) to force a collection (possibly with zero or one elements) return:

$res = @(ls | %{$_.Name} | ?{$_.Contains("Prog")})

Note the difference between these two results:

PS C:\> ConvertTo-Json -InputObject @(1)
[
    1
]
PS C:\> @(1)|ConvertTo-Json
1
PS C:\>

The point is that the 'unboxing' is being done by the pipe operation. ConvertTo-Json still sees the object as an array if we use InputObject rather than piping.

This has been resolved in PowerShell v3:

http://blogs.microsoft.co.il/blogs/scriptfanatic/archive/2012/03/19/Counting-objects-in-PowerShell-3.0.aspx

On a side note, you can find if a name contains something using a wildcard:

PS> ls *og*