[FIX] conditional_formatting: handle zero color scale midpoint

[dhrp-odoo]

Description:

A color scale midpoint value of 0 was treated as absent because the evaluation logic checked the parsed midpoint with truthiness.

As a result, three-point color scales with a midpoint at 0 were evaluated as two-point scales, interpolating directly from minimum to maximum color. This made the midpoint cell use a mixed color instead of the configured midpoint color.

Task: 6312961

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

[robodoo]