[fix] Item group test cases

[rohitwaghchaure]

Issue

ERROR: test_merge_leaves (erpnext.setup.doctype.item_group.test_item_group.TestItem)
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Traceback (most recent call last):
  File "/Users/rohitwaghchaure/Documents/erpnext/frappe-bench/apps/erpnext/erpnext/setup/doctype/item_group/test_item_group.py", line 195, in test_merge_leaves
    self.test_basic_tree(records=records_to_test)
  File "/Users/rohitwaghchaure/Documents/erpnext/frappe-bench/apps/erpnext/erpnext/setup/doctype/item_group/test_item_group.py", line 22, in test_basic_tree
    ["lft", "rgt", "parent_item_group"])
TypeError: 'NoneType' object is not iterable

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ERROR: test_move_group_into_another (erpnext.setup.doctype.item_group.test_item_group.TestItem)
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Traceback (most recent call last):
  File "/Users/rohitwaghchaure/Documents/erpnext/frappe-bench/apps/erpnext/erpnext/setup/doctype/item_group/test_item_group.py", line 90, in test_move_group_into_another
    self.test_basic_tree()
  File "/Users/rohitwaghchaure/Documents/erpnext/frappe-bench/apps/erpnext/erpnext/setup/doctype/item_group/test_item_group.py", line 22, in test_basic_tree
    ["lft", "rgt", "parent_item_group"])
TypeError: 'NoneType' object is not iterable