It is possible to create an invalid entry inside includedFolders, for example:
by creating some folder, adding it to config and then deleting it.
If such entry appears inside config, for example, IndexCleaner will go mad:
it calculates the id of each entry from the config (which for non-existent
file resolves to 0), and then calls tr.removeRecursively(0, shouldDelete).
Which is not something what we want, as 0 can be misintepreted as tree root.
This patch omits invalid entries when building folders cache inside config,
and adds unit test for such case