[C] Fixes setting to Null Binding Context in Picker

[StephaneDelcroix]

Description of Change

This is a retargeted version of #2833. See discussion, and reviews, over there

In the UpdateSelectedItem sends the real selectedIndex

Bugs Fixed

• Fixes Picker.cs Line 215 Throws Argument Out Of Range Exception when Binding Context gets set to Null #2499
• Fixes Picker still attempts to set SelectedItem when BindingContext goes to null #2815
• closes [Core] Fixes setting to Null Binding Context in Picker #2833

API Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense