HPCC-11932 Roxie attach causing Roxie cluster to load empty package

Better for writeCache to clear the cache if it does not exist - otherwise
cache file will never remove unwanted entries.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6255 from richardkchapman/roxie-losing-state

HPCC-11932 Roxie attach causing Roxie cluster to load empty package

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>