Improve parameter detection and errors

[leggetter]

For example, passing an empty string for the Hookdeck Signing Secret results in the following upon the call to verifyWebhookSignature:

DOMException [DataError]: Zero-length key is not supported
    at Object.hmacImportKey (node:internal/crypto/mac:103:15)
    at SubtleCrypto.importKey (node:internal/crypto/webcrypto:635:10)
    at eval (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:42:32)
    at Generator.next (<anonymous>)
    at eval (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:31:71)
    at new Promise (<anonymous>)
    at __awaiter (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:13:12)
    at createSHA256Signature (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:40:58)
    at eval (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:120:38)
    at Generator.next (<anonymous>)
    at eval (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:31:71)
    at new Promise (<anonymous>)
    at __awaiter (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:13:12)
    at verifyWebhookSignature (webpack-internal:///(rsc)/./node_modules/@hookdeck/sdk/webhooks/helpers.js:100:88)
    at POST (webpack-internal:///(rsc)/./app/webhooks/vote/route.ts:16:124)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async /Users/leggetter/hookdeck/git/next-supabase-vote/node_modules/next/dist/compiled/next-server/app-route.runtime.dev.js:6:62609

The empty string value should be caught before the signature generation is attempted.