fix uninitialized arguments

[ShiCheng-Lu]

We had a uninitialized variable (trackingOptions) at startTrip, which caused the dict pointer to be valid as it's uninitialized pointer, and trying to read properties of this "dict" causes some problems.

so also taking this chance to fix all other uninitialized variables. (and clean up some of the variable assignments so it's clear the variables will be valid)

[Copilot]

Pull request overview

This PR fixes uninitialized variable issues in the iOS Flutter plugin by initializing local variables at declaration time instead of relying on conditional initialization. The changes improve code safety by ensuring all variables have defined values before use.

Changes:

• Replaced conditional initialization patterns with inline initialization using default values
• Changed pointer variables (CLLocation, RadarTrackingOptions) to initialize with nil
• Changed primitive int variables to initialize with their default values (1000 for radius, 10 for limit)
• Changed enum variables (RadarRouteUnits) to initialize with default enum values (RadarRouteUnitsImperial)

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[cameron-radar]

I don't know enough about objc to review this properly 😅 . In c# variables are automatically "initialized" to their default values so unless it's different in objc all these changes would seem redundant to me