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_test.go

@@ -360,6 +360,24 @@ func TestModuleConfig_toSysContext(t *testing.T) {
 				sysfs.Adapt(testFS2), // fs
 			),
 		},
+		{
+			name:  "WithFS nil",
+			input: base.WithFS(nil),
+			expected: requireSysContext(t,
+				math.MaxUint32, // max
+				nil,            // args
+				nil,            // environ
+				nil,            // stdin
+				nil,            // stdout
+				nil,            // stderr
+				nil,            // randSource
+				&wt, 1,         // walltime, walltimeResolution
+				&nt, 1, // nanotime, nanotimeResolution
+				nil, // nanosleep
+				nil, // osyield
+				nil, // fs
+			),
+		},
 		{
 			name:  "WithRandSource",
 			input: base.WithRandSource(rand.Reader),