Update copy_back.py

[icy1900]

there is no key data_dir in options_obj.backup_options,
self.backup_options.get("xtra_options") or "" is used to prevent get a None after the --copy-back option, like /usr/bin/xtrabackup --copy-back None --target-dir
option --data_dir for xtrabackup should be --datadir

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.06%.

Quality metrics	Before	After	Change
Complexity	1.56 ⭐	1.61 ⭐	0.05 👎
Method Length	42.08 ⭐	42.25 ⭐	0.17 👎
Working memory	5.62 ⭐	5.62 ⭐	0.00
Quality	85.45% ⭐	85.39% ⭐	-0.06% 👎

Other metrics	Before	After	Change
Lines	147	147	0

Changed files	Quality Before	Quality After	Quality Change
mysql_autoxtrabackup/backup_prepare/copy_back.py	85.45% ⭐	85.39% ⭐	-0.06% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

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