system variables cannot have nil defaults

[jycor]

related: #8585 (comment)

[fulghum]

LGTM

[coffeegoddd]

@jycor DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
8bc92fb	ok	5937457

version	total_tests
8bc92fb	5937457

correctness_percentage
100.0

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.06	1.33
batching	batch sql	10000	1	0.08	1.38
batching	by line sql	10000	1	0.07	1.57
blob	1 blob	200000	1	0.88	3.7	3.65
blob	2 blobs	200000	1	0.89	4.2	4.31
blob	no blob	200000	1	0.88	2.31	2.03
col type	datetime	200000	1	0.8	2.97	2.86
col type	varchar	200000	1	0.69	3.19	2.75
config width	2 cols	200000	1	0.77	2.42	1.96
config width	32 cols	200000	1	1.77	2.07	2.44
config width	8 cols	200000	1	0.99	2.23	1.92
pk type	float	200000	1	0.82	2.28	2.23
pk type	int	200000	1	0.76	2.43	2.07
pk type	varchar	200000	1	1.47	1.67	1.43
row count	1.6mm	1600000	1	5.47	2.84	2.32
row count	400k	400000	1	1.45	2.61	2.13
row count	800k	800000	1	2.82	2.74	2.23
secondary index	four index	200000	1	3.5	1.36	1.07
secondary index	no secondary	200000	1	0.88	2.36	2.03
secondary index	one index	200000	1	1.09	2.35	2.01
secondary index	two index	200000	1	1.93	1.76	1.4
sorting	shuffled 1mm	1000000	0	4.92	2.76	2.34
sorting	sorted 1mm	1000000	1	4.96	2.78	2.25

[github-actions]

@coffeegoddd DOLT

name	detail	mean_mult
dolt_blame_basic	system table	1.1
dolt_blame_commit_filter	system table	2.93
dolt_commit_ancestors_commit_filter	system table	0.65
dolt_commits_commit_filter	system table	1.1
dolt_diff_log_join_from_commit	system table	2.04
dolt_diff_log_join_to_commit	system table	2.07
dolt_diff_table_from_commit_filter	system table	1.21
dolt_diff_table_to_commit_filter	system table	1.21
dolt_diffs_commit_filter	system table	1.06
dolt_history_commit_filter	system table	1.37
dolt_log_commit_filter	system table	1.11