Infinispan file stores without an explicit location all share the same directory

When a cache configuration declares a file store without a location (like the default lesscss.skinfiles.cache configuration, with path=""), XWiki is supposed to put it in a dedicated directory <temporary directory>/cache/<cache configuration id>.

Because of an inverted condition in AbstractCacheConfigurationLoader#createTempDir() (getConfigurationId() == null instead of != null), the configuration id is never appended, so all those file stores share the same <temporary directory>/cache directory (for both data and index).

With the default configuration it has no visible effect since only one cache has a file store, but several caches with a file store and no explicit location would share the same Infinispan store files.