feat(firestore): Add expressions ifNull and coalesce

[milaGGL]

Ported from firebase/firebase-js-sdk#9753

[gemini-code-assist]

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for GitHub and other Google products, sign up here.

[milaGGL]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds support for the Pipeline expressions ifNull and coalesce to the Firestore Swift SDK, including the necessary implementation and integration tests. I have reviewed the changes and the documentation parameter name for coalesce should be updated to match the function signature.

[yvonnep165]

Changes look good to me except the minor suggested formatting fix and the coalesce test fails as it hasn't been pushed to prod at this moment