[FIX] hr_timesheet_invoice: warning in report generation

When generating the report 'Timesheet Profit', got a warning "The domain term '('user_id', '=', [...])' should use the 'in' or 'not in' operator."
This warning is due to the use of the '=' operator to compare the field 'user_id' while the reports sends a list of ids.
Fallback to still accept a single id in case of customised reports.