describe dependents rejects missing optional targets

## Problem

`atmos describe dependents` rejects a missing target before it can apply an optional reverse dependency declaration.

## Reproduction

`./build/atmos --chdir tests/fixtures/scenarios/dependencies-components-inheritance describe dependents network-baseline --stack dev --format json`

The `dev/vpc` component declares `network-baseline` with `required: false`, but the command returns `invalid component` instead of an empty JSON result.

## Expected behavior

A target referenced only by optional dependencies should return an empty dependent list when the target is absent.

Start with the `atmos describe dependents` command using the reproduction in `tests/fixtures/scenarios/dependencies-components-inheritance`. Trace how the missing `network-baseline` target is validated against the optional declaration in `dev/vpc`; done means the command returns an empty JSON dependent list instead of `invalid component`.

Written by the indexing model from the issue text.