fix: omit empty optional fields from payloads

[Luffy-D-Zoro]

What

Fixes #14

This PR fixes validation failures when optional form fields are submitted empty.
Previously, some optional frontend fields were sent as null:

• notes in PostRide.jsx
• maxFare in PostIntent.jsx

The backend schemas treat these fields as optional, not nullable, so sending null caused validation to fail.

This PR changes the empty optional field values from null to undefined.

When the request payload is stringified, fields with undefined values are omitted from the JSON body. This matches the backend schemas where these fields are optional.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Proof of work

Tested locally:

• Posting a ride without notes works.
• Posting a ride with notes works.
• Posting an intent without max fare works.
• Posting an intent with max fare works.

Checklist

• I claimed this issue before starting work
• Branch name follows the convention (feature/, fix/, docs/)
• Commits follow Conventional Commits format
• CI is passing (lint, prettier, build)
• I have self-reviewed my own code
• Any non-obvious decisions are explained with a comment in the code