Do not make dolt_diff table indexes on nil schemas

[angelamayxie]

Calling MakeDiffTableIndex on a nil schema was causing a panic. This would happen if a table existed on one branch but not on the other.

[coffeegoddd]

@angelamayxie DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	2.3	2.3	0.0
groupby_scan	132.49	134.9	1.82
index_join	1.89	1.89	0.0
index_join_scan	1.34	1.34	0.0
index_scan	215.44	215.44	0.0
oltp_point_select	0.26	0.26	0.0
oltp_read_only	5.09	5.09	0.0
select_random_points	0.55	0.55	0.0
select_random_ranges	0.64	0.64	0.0
table_scan	207.82	207.82	0.0
types_table_scan	467.3	467.3	0.0

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.21	6.21	0.0
oltp_insert	3.19	3.13	-1.88
oltp_read_write	11.45	11.45	0.0
oltp_update_index	3.36	3.3	-1.79
oltp_update_non_index	3.02	3.02	0.0
oltp_write_only	6.32	6.32	0.0
types_delete_insert	6.91	6.91	0.0

[coffeegoddd]

@angelamayxie DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	45.79	44.17	-3.54

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	75273d6	52.47	dolt	430f1d6	52.86	0.74

[coffeegoddd]

@angelamayxie DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
430f1d6	ok	5937471

version	total_tests
430f1d6	5937471

correctness_percentage
100.0