maple_tree: add a test checking storing null

maple_tree: add a test checking storing null

Add a test to assert that, when storing null to am empty tree or a
single entry tree it will not result into:

  * a root node with range [0, ULONG_MAX] set to NULL
  * a root node with consecutive slot set to NULL

[akpm@linux-foundation.org: work around build error (mas_root)]
Link: https://lkml.kernel.org/r/20241031231627.14316-6-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>