Fix nil pointer panic in workspace table Update and Delete methods

[nullun]

When StatementBegin encounters an error (e.g., table not found in staging root), it stores the error in wtu.err but leaves tableWriter as nil. The Update and Delete methods were dereferencing tableWriter before checking if it was nil, causing a panic.

This fix adds an early return to check for errors from StatementBegin before attempting to use tableWriter, preventing the nil pointer dereference.

[jycor]

@nullun Thank for your the contribution.
The change looks good, but do you have a way to reproduce the panic?

[nullun]

@nullun Thank for your the contribution. The change looks good, but do you have a way to reproduce the panic?

It happens when doing dolt add -p on a table that only exists in working. It's what lead me on to my other PR (#10591), but I thought it would be better to keep this small fix on its own.

[jycor]

LGTM

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.05	2
batching	batch sql	10000	1	0.06	2.17
batching	by line sql	10000	1	0.06	2
blob	1 blob	200000	1	0.88	4.25	4.31
blob	2 blobs	200000	1	0.86	4.77	4.66
blob	no blob	200000	1	0.88	2.77	2.58
col type	datetime	200000	1	0.81	2.63	2.46
col type	varchar	200000	1	0.67	4	3.91
config width	2 cols	200000	1	0.74	2.77	2.62
config width	32 cols	200000	1	1.72	3.45	3.02
config width	8 cols	200000	1	0.96	2.89	3.05
pk type	float	200000	1	0.82	2.59	2.6
pk type	int	200000	1	0.79	2.62	2.41
pk type	varchar	200000	1	1.56	1.63	2.09
row count	1.6mm	1600000	1	5.77	2.91	2.69
row count	400k	400000	1	1.42	2.91	2.71
row count	800k	800000	1	2.92	2.87	2.64
secondary index	four index	200000	1	3.4	1.37	1.12
secondary index	no secondary	200000	1	0.88	2.77	2.58
secondary index	one index	200000	1	1.08	2.63	2.31
secondary index	two index	200000	1	1.87	1.86	1.68
sorting	shuffled 1mm	1000000	0	5.4	2.88	2.78
sorting	sorted 1mm	1000000	1	5.49	2.85	2.77

[github-actions]

@coffeegoddd DOLT

name	detail	mean_mult
dolt_blame_basic	system table	1.25
dolt_blame_commit_filter	system table	1.19
dolt_commit_ancestors_commit_filter	system table	0.61
dolt_commits_commit_filter	system table	1.05
dolt_diff_log_join_from_commit	system table	2.92
dolt_diff_log_join_to_commit	system table	2.91
dolt_diff_table_from_commit_filter	system table	1.22
dolt_diff_table_to_commit_filter	system table	1.25
dolt_diffs_commit_filter	system table	1.03
dolt_history_commit_filter	system table	1.48
dolt_log_commit_filter	system table	1.1