Convert undefined custom properties to empty string

[xiao-lix]

To address this issue: Convert undefined to blank in customDimensions #1471

Solution: Add a sender configuration convertUndefined that customer can set custom defined value and convert undefined field to the value they need.

Sample Usage:

• AI set up:

const ai = new ApplicationInsights({
    config: {
        instrumentationKey: '<YOUR_IKEY>',
        maxBatchInterval: 100,
        disableFetchTracking: false,
        convertUndefined: 'test'
    }
});
ai.loadAppInsights();

• send a event track:
  ai.trackEvent({name: "test-event", properties: {test2: undefined}});
• without the configuration convertUndefined payload :
  
• with configuration convertUndefined payload: