Introduce flags for nullable type support and strict null checking in JS (#6763)
Introduce flags for nullable type support and strict null checking in JS (#6763)

Closes #6763

COPYBARA_INTEGRATE_REVIEW=https://github.com/polymerlabs/arcs/pull/6763 from Cypher1:nullableFieldSupportFlag 80c4a0d5a62e0f9467536864209487b199123231
PiperOrigin-RevId: 347852848

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs