Validate the path, not the parameter name

[SimonCropp]

Guard.FileExists called AgainstEmpty(argumentName, path) - arguments reversed,
so the empty check ran against the literal parameter name, which is never empty.
An empty path fell straight through to ArgumentException("File not found. Path:
") with no ParamName, which names neither the argument nor the file.

The same commit stops OsSettingsResolver dereferencing a null PATH. env -i and
some service launchers do start a process without one, and this runs in a static
constructor, so the NullReferenceException is permanent for the process. An
empty PATH just means no tool is found that way, which is a perfectly good
answer.

No test for the PATH half: it is read once by a type initialiser, so it cannot
be exercised twice in one process.