Add support for default value on a dimension link when there are nulls after a join

[shangyian]

Summary

When using left or right joins for dimension links, unmatched rows produce NULL values for dimension columns. This can be problematic for downstream consumers that expect non-null values.

The PR adds support for setting default_value on a dimension link, which enables users to specify a fallback value (e.g., "Unknown", "N/A") that will be used in place of NULL. When a dimension link has a default_value configured, dimension columns will be wrapped in COALESCE(column, 'default_value') in the generated SQL, providing a fallback value when the join doesn't match.

The changes are needed across various interfaces:

Database + Models

• Added default_value column to DimensionLink database model
• Added database migration for the new column

YAML Support

dimension_links:
  - dimension_node: users
    join_type: left
    join_on: "events.user_id = users.id"
    default_value: "Unknown"

SQL Generation

• V2 SQL Builder: Updated build_dimension_attribute to wrap dimension columns in coalesce
• V3 SQL Builder: Updated build_select_ast() to apply coalesce

Generates queries like

SELECT
  COALESCE(users.name, 'Unknown') AS user_name
FROM events
LEFT JOIN users ON events.user_id = users.id

GraphQL

Added defaultValue field to DimensionLink type in the GraphQL schema

UI

Added "Default Value" form field to the complex dimension link popover.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

[netlify]

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