Preserve nil values during RN <-> Obj-C <-> Swift bridge to avoid conversion to empty string

[perarnaudalain]

Fix issue where nil optional values were incorrectly converted to empty strings ("") when passed through the React Native → Objective-C → Swift bridge. Now properly preserves nullability.

[lorenc-tomasz]

Good point :)

[lorenc-tomasz]

I suggest:

    @objc(setCustomDimension:withValue:withResolver:withRejecter:)
    func setCustomDimension(index: NSNumber, value: String?, resolve: RCTPromiseResolveBlock, reject: RCTPromiseRejectBlock) -> Void {
        guard let tracker = self.tracker else {
            reject("not_initialized", "Matomo not initialized", nil)
            return
        }

        if let unwrappedValue = value {
            tracker.setDimension(unwrappedValue, forIndex: index.intValue)
        } else {
            tracker.remove(dimensionAtIndex: index.intValue)
        }
        
        resolve(nil)
    }

[lorenc-tomasz]

Fixed within: #7