Join-Path routes a drive-qualified path through the PowerShell provider, so
it returns null when that drive has gone away mid-cycle, and every consumer
then bound the null straight into Test-Path -LiteralPath and crashed the
entry with a message that names neither the entry nor the path.
Replace the six Join-Path $InstallerRoot sites with Join-InstallerPath, which
does plain string math, and have all six callers treat a null resolve as
"not found" and log it. Only the PS1 branch had a guard before, and it
covered a null Script value, not a null resolved path.
The helper deliberately avoids [IO.Path]::Combine: its separator and
rooted-path rules follow the host platform, so behaviour cannot be verified
off-Windows. It also rejects rooted values ("\x", "D:\x", "\\server\share"),
which Combine would have resolved outside the share root. Checked all 36
path values across the share manifests: none are rooted today.
Behaviour verified against the real manifest values (forward slashes,
backslashes, trailing-slash root, null, whitespace, rooted, UNC).
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