fix: bug preventing pos closing entry creation

[Starktail-Christo]

Description

Fixed a bug that prevented POS Closing entries from being created

Type of change

• validate_pos_invoices was looking for an array, instead of a string

Tests

• Unit Tests have been updated or added, as required
• UI Tests have been updated or added, as required

Checklist:

• My code follows Naming Guidelines (DocType, Field and Variable naming)
• No Form changes / My code follows the Form Design Guidelines
• My code follows the Coding Standards of this project
• My code follows the Code Security Guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas / No comments necessary
• I have made corresponding additions/changes to the documentation
• All business logic and validations are on the server-side / No business logic or validation changes
• No patches are necessary / Migration Patches have been added to the correct subdirectory of /patches and patches.txt have been updated

User Experience:

User experiance remains the same.

[Starktail-Christo]

@dlw8
Its a very small change. But is quite urgent.