[FIX]product_configurator : fix issue- IndexError: tuple index out of range.

[bizzappdev]

Impacted versions: 10.0

Steps to reproduce:
1) create configurable product-template
2) add variants
3) add restriction and keep domain-lines(RULES) blank
4) go to sale > create sale order > and try to configure product created above

Current behaviour: it will raise an error with traceback

File "/home/shruti/odoo/v10/odoo_product_configurator/git_hub/odoo-product-configurator/product_configurator_wizard/wizard/product_configurator.py", line 735, in action_next_step
self.value_ids.ids, active_cfg_line_id)
File "/home/shruti/odoo/v10/odoo_product_configurator/git_hub/odoo-product-configurator/product_configurator/models/product.py", line 90, in get_adjacent_steps
open_step_lines = self.get_open_step_lines(value_ids)
File "/home/shruti/odoo/v10/odoo_product_configurator/git_hub/odoo-product-configurator/product_configurator/models/product.py", line 68, in get_open_step_lines
value_ids)
File "/home/shruti/odoo/v10/odoo_product_configurator/git_hub/odoo-product-configurator/product_configurator/models/product.py", line 433, in values_available
domains = config_lines.mapped('domain_id').compute_domain()
File "/home/shruti/odoo/v10/odoo_product_configurator/git_hub/odoo-product-configurator/product_configurator/models/product_config.py", line 47, in compute_domain
(lines[-1:].attribute_id.id,
File "/home/shruti/odoo/v10/odoo/odoo/models.py", line 5251, in getitem
return self._browse((self._ids[key],), self.env)
IndexError: tuple index out of range

Expected behaviour: No traceback should be there

[codecov-io]

Codecov Report

Merging #164 into 10.0 will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             10.0    #164      +/-   ##
=========================================
+ Coverage   62.17%   62.2%   +0.02%     
=========================================
  Files          14      14              
  Lines        1351    1352       +1     
=========================================
+ Hits          840     841       +1     
  Misses        511     511

Impacted Files	Coverage Δ
product_configurator/models/product_config.py	76.43% <100%> (+0.12%)	⬆️

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[bizzappdev]

@PCatinean can you please check.