Hotfix for 17c9c034

After previous patch being applied ASAN detects following behaviour:

box/key_def.c:295:39: runtime error: load of value 77, which is not a valid value for type 'bool'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior

It happens since field exclude_null in key_part_def was uninitialized
and passed to key_def_new() (see constructAutomaticIndex()). So let's fix
this and set exclude_null field to default value 'false'.

Follow-up #4480