Ensure nil is accepted as FS config (#1182)

In previous releases, passing a `nil` value as an FS config
did not cause any error. However, in 1.0.0-rc.1 this leads
to the creation of an invalid `adapter{fs: nil}`, which
eventually leads to a panic (nil):

    (f *FileEntry) Stat(st *platform.Stat_t) =>
        (r *lazyDir) file() =>
            r.fs.OpenFile(".", os.O_RDONLY, 0)

with fs == nil

The backwards-compatible fix is to make Adapt()
return UnimplementedFS, and ensuring `nil` is a valid value
that config is able to handle.

However, we may want to consider returning an error somewhere,
because configuring a nil FS may be unintended.

Signed-off-by: Edoardo Vacchi <evacchi@users.noreply.github.com>

config.go

@@ -667,7 +667,11 @@ func (c *moduleConfig) WithEnv(key, value string) ModuleConfig {
 
 // WithFS implements ModuleConfig.WithFS
 func (c *moduleConfig) WithFS(fs fs.FS) ModuleConfig {
-	return c.WithFSConfig(NewFSConfig().WithFSMount(fs, ""))
+	var config FSConfig
+	if fs != nil {
+		config = NewFSConfig().WithFSMount(fs, "")
+	}
+	return c.WithFSConfig(config)
 }
 
 // WithFSConfig implements ModuleConfig.WithFSConfig