BaseTools: Improve the method of checking queue empty

BaseTools: Improve the method of checking queue empty

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2807

The Queue.empty() method is not reliable in the multiple
process runtime environment. This patch uses a new method
to check if all modules are processed and workers need
to be stopped. That is to add a None item at the bottom
of the queue. Worker check if it gets that None item to
know if all the module is processed.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Lucy Yan <lucyyan@google.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>