Some generated extensions are missing nullable annotations

[atifaziz]

The following method overloads and implementations that are generated via T4 templates are missing nullable annotations even though the entire project has the nullable context enabled:

• ToDelimitedString
• Aggregate
• Fold
• Cartesian

This happens because of the following (documented) quirk:

Older code generation strategies may not be nullable aware. Setting the project-level nullable context to "enable" could result in many warnings that a user is unable to fix. To support this scenario any syntax tree that is determined to be generated will have its nullable state implicitly set to "disable", regardless of the overall project state.