protocols/catalog: skip NULL-spec rows in LoadAll* queries

[SeanWhelan]

Description:

flowctl catalog delete --name <test> (and any publication that deletes a test) currently fails with:

test:N> FATA fatal error err="extracting from build: validating spec : missing Name"
Error: failed with status: testFailed

The Rust build layer represents a deletion as a built_* row with spec NULL (per BuiltRow::is_delete in built.rs). The Go-side LoadAll* queries in build_load.go did not filter those rows, so flowctl-go api test would Scan a NULL spec into an empty byte slice, Unmarshal it into a zero-value TestSpec, and trip TestSpec.Validate() with missing Name.

Fix: add WHERE spec IS NOT NULL to all four LoadAll* queries (collections, captures, materializations, tests). The same defect was latent in the other three loaders even though the visible failure was only in tests, since the test phase is the one that per-row-validates inside loadSpecs.

Workflow steps:

Repo before fix:

cat > flow.yaml <<'YAML'
collections:
  acmeCo/widgets:
    schema: { type: object, properties: { id: { type: string } }, required: [id] }
    key: [/id]
tests:
  acmeCo/widget-test:
    - ingest: { collection: acmeCo/widgets, documents: [{ id: "w1" }] }
    - verify: { collection: acmeCo/widgets, documents: [{ id: "w1" }] }
YAML
flowctl catalog publish --source flow.yaml --auto-approve
flowctl catalog delete --name acmeCo/widget-test
# → testFailed: validating spec : missing Name

Documentation links affected:

(list any documentation links that you created, or existing ones that you've identified as needing updates, along with a brief description)

Notes for reviewers:

(anything that might help someone review this PR)

[jgraettinger]

LGTM

As a nit, LLMs tend to overly-extend comments to document the conditions of a change, including discussion of historical conditions that are no longer true, or handling which is obvious just by reading the code itself. In this case, I would have left the function comments as they are, and just added the WHERE guard.