bug#40949: 26.3; substitute-env-in-file-name: Undefined variables not substituted

[Marcus Harnisch]

With commit ee041f2d07b6ed485dc34c115588f973f046c9d4 the function
`substitute-env-in-file-name' was introduced.

The function substitutes environment variable references in filenames
with the help of `substitute-env-vars'. On systems that are neither
windows-nt nor ms-dos, the optional argument to that function is set to
t with the consequence that references to undefined variables are not
substituted. This is unexpected, IMHO.
Expected behaviour would match that of shell expansion, which
substitutes undefined variables with the empty string.

Cheers,
Marcus