fix: enabled returns undefined instead of false when DEBUG env var is absent

[JSap0914]

Problem

When DEBUG is not set (or enable() is called with a non-string value such as undefined), instance.enabled incorrectly returns undefined instead of false.

Root cause

Inside enable(), createDebug.namespaces is assigned the raw argument:

createDebug.namespaces = namespaces; // can be undefined

Each debug instance caches its enabled state with:

let namespacesCache;   // initialised to undefined
let enabledCache;      // initialised to undefined

get: () => {
  if (namespacesCache !== createDebug.namespaces) { // undefined !== undefined → false!
    namespacesCache = createDebug.namespaces;
    enabledCache = createDebug.enabled(namespace);  // never reached on first access
  }
  return enabledCache; // returns undefined
}

Because both namespacesCache and createDebug.namespaces start as undefined, the cache-miss guard evaluates to false on the very first access, createDebug.enabled() is never called, and enabledCache remains undefined.

Observable effect

// No DEBUG env var set
const debug = require('debug');
const log = debug('myapp');

log.enabled              // → undefined  (expected: false)
log.enabled === false    // → false       (expected: true)
typeof log.enabled       // → 'undefined' (expected: 'boolean')

The common guard if (!debug.enabled) return; still works because !undefined === true, but strict comparisons and TypeScript type-checks fail.

Fix

Normalise createDebug.namespaces to an empty string when enable() receives a non-string argument, consistent with how the split step already handles non-string input:

createDebug.namespaces = typeof namespaces === 'string' ? namespaces : '';

This ensures the cache-miss guard fires on the first read after startup, so enabledCache is properly initialised.

Test

A new failing test is added that re-requires debug with no DEBUG env var and asserts typeof log.enabled === 'boolean' and log.enabled === false.

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