GH-272 Fix Null Exception on Location Updates

[jamesmontemagno]

Description of Change

Check for null :)

Bugs Fixed

• Related to issue Null Reference Exception inside of Geolcoation on iOS on Simulator #272

Provide links to issues here. Ensure that a GitHub issue was created for your feature or bug fix before sending PR.

API Changes

Changed:
Ensure on each platform that null is checked.

Added helpers on UWP to simplify things.

Behavioral Changes

Describe any non-bug related behavioral changes that may change how users app behaves when upgrading to this version of the codebase.

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation (see walkthrough)

[dend]

✅ Validation status: passed

File	Status	Preview URL	Details
Xamarin.Essentials/Geolocation/Geolocation.android.cs	✅Succeeded
Xamarin.Essentials/Geolocation/Geolocation.ios.cs	✅Succeeded
Xamarin.Essentials/Geolocation/Geolocation.uwp.cs	✅Succeeded
Xamarin.Essentials/Types/LocationExtensions.uwp.cs	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.