fix(inventory): BACK-786 Fallback to the default backorder message ID when backorder message ID is null

[agchan12]

What?

• Remove null guard from simple/complex and picklist products.
  • backorder_message_ids from products can be null. When they are null it means they are the current default. We still need to display the backorder message when the ID is null.

Requirements

• CHANGELOG.md entry added (required for code changes only)

Tickets / Documentation

Add links to any relevant tickets and documentation.

• BACK-786

Screenshots (if appropriate)

Before

Setup
Product 103 is set to use the default backorder message 1
Backorder message is set to display in the PDP

BACK-786-before.mov

Explanation
On the PDP, the backorder message does not display because of the null guard if the backorder message is null

After

Setup
Product 103 is set to use the default backorder message 1
Backorder message is set to display in the PDP

BACK-786-after.mov

Explanation
On the PDP, the backorder message does display when the backorder message is set null

ping @animesh1987 @bigcommerce/team-trac

---

Note

Low Risk
Localized PDP/picklist display logic with no auth or checkout impact; behavior change only affects when backorder message text appears.

Overview
PDP backorder copy now resolves through a shared findByBackorderMessageIdOrDefault helper instead of requiring a non-null backorder_message_id.

When a product (or picklist-linked product) has backorder_message_id null—meaning “use the store default”—or points at a deleted message, the theme shows the message marked is_default in backorderMessages. The main PDP backorder text (updateBackorderMessage) and picklist backorder suffixes (lookupBackorderMessage) both use this path; earlier null guards that skipped lookup entirely are removed so default messaging can appear. If the default record exists but its message text is empty, nothing is shown (unchanged empty-state behavior).

Unit coverage was added for PDP and picklist fallback, dangling IDs, and blank defaults.

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[animesh1987]

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