fix: avoid edge case error message in log

[FHenry]

in some edge case I've get Error in dolibarr log

2024-10-29 14:57:01 ERR 89.91.85.217 Error, you call sanitizeVal() with a bad value for the check type. Data can't be sanitized.
2024-10-29 15:00:42 ERR 89.91.85.217 DoliDBMysqli::query SQL Error query: SELECT t.rowid,t.date_creation,t.tms,t.import_key,t.status,t.role,t.gender,t.civility,t.firstname,t.lastname,t.job
,t.email,t.phone,t.society_name,t.signature_date,t.signature_location,t.signature_comment,t.element_id,t.element_type,t.module_name,t.signature,t.stamp,t.signature_url,t.transaction_url,t.last_em
ail_sent_date,t.attendance,t.object_type,t.fk_object FROM llx_saturne_object_signature as t WHERE 1 = 1 AND t.entity IN (1) AND fk_object = AND status > 0 AND object_type = "user" AND role = "Us
erSignature" LIMIT 1
2024-10-29 15:00:42 ERR 89.91.85.217 DoliDBMysqli::query SQL Error message: DB_ERROR_SYNTAX You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server v
ersion for the right syntax to use near 'AND status > 0 AND object_type = "user" AND role = "UserSignature" LIMIT 1' at line 1

if fk_object is not defined => better to do not run SQL query => it's better than have error in log