install.ps1 aborts on upgrade: File.Replace rejects the null backup path #1354

Description

Upgrading an existing Windows install with the documented one-liner aborts at the last step, after the
binary and PATH are already in place, and leaves ~/.config/thinkrail/install.json describing the
previous version.

irm https://raw.githubusercontent.com/JetBrains/thinkrail/main/install.ps1 | iex

from 0.3.2 to 0.4.0:

ThinkRail 0.4.0 (stable) installed.
PATH:           already configured
Invoke-Expression: Exception calling "Replace" with "3" argument(s): "The path is empty. (Parameter 'path')"

thinkrail --version reports 0.4.0, but the closing Run: / Update later: / Uninstall: lines never
print and the metadata still reports 0.3.2.

Steps to reproduce

1. Install 0.4.0 with the one-liner.
2. Run the same one-liner again over that install.
3. The second run ends with the exception above.

Minimal check, independent of the installer:

$d = Join-Path $env:TEMP ([guid]::NewGuid().ToString('N'))
New-Item -ItemType Directory $d | Out-Null
$new = Join-Path $d 'new'; $dst = Join-Path $d 'old'
Set-Content $new 'a'; Set-Content $dst 'b'
[System.IO.File]::Replace($new, $dst, $null)                 # throws
[System.IO.File]::Replace($new, $dst, [NullString]::Value)   # works

Expected behavior

The second install replaces install.json with the new version and prints its closing lines.

Actual behavior

Install-ThinkRail writes the metadata with:

if (Test-Path -LiteralPath $metaFile) {
    [System.IO.File]::Replace($metaTemp, $metaFile, $null)
} else {
    [System.IO.File]::Move($metaTemp, $metaFile)
}

PowerShell binds $null to the string parameter as an empty string, so File.Replace receives ""
as the backup path instead of "no backup" and rejects it. PowerShell 7 reports The path is empty;
Windows PowerShell 5.1 reports The path is not of a legal form for the same call.

Only the Replace branch runs when install.json already exists, so a first install succeeds and every
install over an existing one fails at the last step.

Fix: pass a real null, [System.IO.File]::Replace($metaTemp, $metaFile, [NullString]::Value).
[NullString]::Value exists in both Windows PowerShell 5.1 and PowerShell 7.

Environment

• OS: Windows 11 x64
• Bun version (bun --version): 1.4.4
• Node version (node --version): v26.13.0
• ThinkRail version / commit: 0.4.0 (stable), upgrading from 0.3.2; PowerShell 7.6.9, also reproduced
  under Windows PowerShell 5.1