[#493] node/config: Change Sub implementation

In some cases viper doesn't interpret `section.value` as a subsection with
`section` name, but value is value still can be accessed through full
pathname.

Fix `Config.Sub` method implementation in order to always interpret
configuration like described above as a subsection. From now method never
returns nil, therefore an additional check has been removed from the `Value`
method.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config/calls_test.go

@@ -21,9 +21,6 @@ func TestConfigCommon(t *testing.T) {
 
 		const nonExistentSub = "non-existent sub-section"
 
-		sub = c.Sub(nonExistentSub)
-		require.Nil(t, sub)
-
 		val = c.Sub(nonExistentSub).Value("value")
 		require.Nil(t, val)
 	})