[BUGFIX] Clear_cache() must not consider page ids lower than 0

DataHandler::clear_cache() is called with a table and an uid that
has been modified to trigger the clear cache commands. To find
siblings, children, parents, etc. the pid value is directly used
to find the affected pages. However, in a workspace context, the
pid is always "-1" which leads to a selection of all page
records of all workspaces. This amount is used to flush all
defined caches by a given tag name and might result in execution
time outs.

To avoid these superfluous cache flushes, page ids lower than 0
are ignored now in this section.

Fixes: #51051
Releases: 6.0, 6.1, 6.2
Change-Id: I03987bbd8c5d3526fb4dfda75b0dd0316fe1d836
Reviewed-on: https://review.typo3.org/24592
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader