Consider possible `null`s in capability objects

[toinehartman]

Describe the bug

Along the lines of #996, but for other capabilities.

1. Update the CF stubs to make all of these @Nullable.
2. Fix all uses, possibly with a generic mechanism to work around null values.

Desktop (please complete the following information):

• Context: VS Code extension
• Rascal Version: 0.13.1-SNAPSHOT