Description

Add explicit inhibitIf and inhibitIfEmpty flow-control helpers to Grafast.

inhibitIf is implemented as a regular step for condition-based inhibition that
cannot be absorbed into dependency flags like inhibitOnNull. inhibitIfEmpty
adds a common helper for inhibiting null, undefined, "", empty arrays, and
objects with no keys.

Also adds tests and documentation updates for the new flow-control APIs.

Performance impact

Low. inhibitIf is an explicit step, so it is less optimizable than
inhibitOnNull, but only affects plans that opt into this behaviour.

Security impact

None.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.