webhook server: conditionally omitting .Branch from template vars breaks task creation with missingkey=error

Problem

internal/webhook/github_filter.go builds the webhook template variable map and adds .Branch only when the value is non-empty:

if eventData.Branch != "" {
    vars["Branch"] = eventData.Branch
}

The task builder and task controller parse templates with .Option("missingkey=error"):

• internal/taskbuilder/builder.go:160
• internal/controller/task_controller.go:874

So any TaskSpawner template that references {{.Branch}} fails to render when the webhook payload does not carry a branch, and task creation aborts with map has no entry for key "Branch".

When .Branch is empty

• issue_comment event posted on a regular issue — GitHub's payload has no head ref because there is no PR
• issue_comment event posted on a pull request when the webhook server has no GitHub API credentials — the lazy enrichGitHubIssueCommentBranch fetch in internal/webhook/github_api.go returns empty and logs "No GitHub credentials configured, cannot enrich issue_comment event with PR branch"
• Push events with no ref string (unusual but possible via synthetic or incomplete deliveries)

Impact

Any TaskSpawner whose taskTemplate.branch is set to {{.Branch}} silently fails to create tasks in the above cases. This is the recommended idiom for PR-scoped spawners — the value is also used by TaskReconciler as the per-branch writer lock — so any spawner following that convention is affected whenever the above conditions hit.

The failure mode is particularly hard to debug:

• Webhook server logs show delivery accepted
• TaskSpawner.status.activeTasks does not increment
• The error only appears in controller logs when it tries to render the template
• No Task object is ever created, so kubectl get tasks shows nothing

Reproduction

1. Deploy kelos with the webhook server enabled but webhookServer.sources.github.githubSecretName unset
2. Create a TaskSpawner with taskTemplate.branch: "{{.Branch}}" and an issue_comment filter
3. Post a matching comment on an open PR
4. Check kelos-controller-manager logs: template render fails with map has no entry for key "Branch"
5. Confirm no Task is created

Same failure is reproducible by posting the trigger comment on an issue instead of a PR, even when credentials ARE configured.

Start with internal/webhook/github_filter.go, then inspect the template parsing in internal/taskbuilder/builder.go:160 and internal/controller/task_controller.go:874. Reproduce the issue with an issue_comment event lacking a branch and verify that a TaskSpawner template referencing .Branch can render and create a Task with an empty branch value.