fix: resolve PHP 8.4 implicitly nullable parameter deprecations

[iruzevic]

PHP 8.4 deprecated implicitly nullable parameters (when a typed parameter has a null default but the type doesn't explicitly include null). Several StellarWP packages are affected:

stellarwp/arrays v1.3.2 — 2 occurrences
stellarwp/assets v1.5.1 — 3 occurrences
stellarwp/container-contract v1.0.4 — 2 occurrences
stellarwp/db v1.3.0 — 3 occurrences

• src/DB/DB.php:409,426 — string $query = null -> ?string $query = null
• src/DB/Database/Exceptions/DatabaseQueryException.php:36 — Throwable $previous = null -> ?Throwable $previous = null
  stellarwp/field-conditions v1.1.1 — 7 occurrences
  stellarwp/models v1.2.3 — 6 occurrences

All fixes follow the same pattern: SomeType $param = null -> ?SomeType $param = null.

No behavior change, only type signature corrections.

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• https://github.com/stellarwp/arrays
• https://github.com/stellarwp/assets
• https://github.com/stellarwp/container-contract
• https://github.com/stellarwp/db
• https://github.com/stellarwp/field-conditions
• https://github.com/stellarwp/models