Settle on either undefined or null for async data's data and error

[DamianGlowala]

Describe the feature

Currently, there is a discrepancy in default values of async data's data and error:

• data is assigned an undefined value (unless a different default value is specified)
• error is assigned a null value

It would be good to consider settling on one of them for consistency, unless there's a justifiable reason for why they have to differ.

Additional information

• Would you be willing to help implement this feature?
• Could this feature be implemented as a module?

Final checks

• Read the contribution guide.
• Check existing discussions and issues.

[danielroe]

We will settle on undefined 🚀 🔥

[TheAlexLichter]

To not forget: This might mean additional behavior changes for getCachedData() too, or an additional check for compat.