[#690] cmd/neofs-node: Fallback to wallet section if node key is not set

Some users want to specify only wallet section in the SN. It is not
possible if `Key` throws panic on empty value. Instead it should
fallback to wallet section. Panic is suitable if node's key is provided
but invalid.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

cmd/neofs-node/config/node/config_test.go

@@ -14,9 +14,8 @@ func TestNodeSection(t *testing.T) {
 	t.Run("defaults", func(t *testing.T) {
 		empty := configtest.EmptyConfig()
 
-		require.PanicsWithError(
+		require.Panics(
 			t,
-			errKeyNotSet.Error(),
 			func() {
 				Key(empty)
 			},