[IndexCleaner] ignore non-existent entries inside config

Summary:
If a folder was added to include/excludeFolders and then removed,
there will be a stale entry in the config, for which filePathToId
yields 0. When passed to removeRecursively, it might reinterpret it
as tree root, which is not something we want (although there are
checks for it inside WriteTransaction)

See also D22502: [FileIndexerConfig] skip invalid entries from included/excludedFolders

Reviewers: Baloo, bruns, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D22557