fix(logger): skip Slack POST when webhook URL is empty

[droplet-rl]

What

Adds an early-return in SlackHook.log (packages/logger/src/logger/SlackTransport.ts) so that when an escalation path resolves to an empty / unset webhook URL, the message is dropped silently instead of being POSTed.

const webhookUrl = this.escalationPathWebhookUrls[info.notificationPath] ?? this.defaultWebHookUrl;

// An empty (or unset) webhook URL means this notification path has been intentionally disabled in the config.
// Drop the message silently rather than POSTing to an empty URL, which would otherwise surface as a transport
// error and escalate to PagerDuty.
if (!webhookUrl) return callback();

Why

This is the enabling change for turning off a noisy Slack notification path from config alone (in UMAprotocol/bot-configs) without code changes per-bot.

Without this guard, blanking a webhook URL in SLACK_CONFIG.escalationPathWebhookUrls would just move the failure: the transport would POST to "", axios would throw, the logger would emit an error, and PagerDutyV2Transport would escalate it — exactly the failure mode currently paging on PD incident Q2925Q7EDE9S6W (managed-oo-polygon-monitor, Slack 429s on the optimistic-oracle path).

Behaviour is unchanged when a webhook URL is present.

Companion PR

Config-side change that actually disables the path lives in UMAprotocol/bot-configs (linked in the Slack thread). Merge this one first so the config change is a clean skip rather than an empty-URL POST.

Triggered from Slack by @lee Poettcker.

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