Fails to look up loader when an extension is single character.

[sttk]

What were you expecting to happen?

Looks up loader successfully when an extension is single character.

What actually happened?

rechoir throws an error of which message is "No module loader found for "undefined". when an extension is single character.

Please give us a sample of your gulpfile for reproduction

// test.js
const config = {                                                                                                                         
  '.coffee': 'coffeescript/register',
  '.co': 'coffeescript/register',
  '.c': 'coffeescript/register',
};
const rechoir = require('rechoir');

console.log(rechoir.prepare(config, './aaa.coffee'));
console.log(require('./aaa.coffee'));

console.log(rechoir.prepare(config, './aaa.co'));
console.log(require('./aaa.co'));

console.log(rechoir.prepare(config, './aaa.c'));
console.log(require('./aaa.c'));

Terminal output / screenshots

$ npm i rechoir coffeescript
$ touch aaa.coffee aaa.co aaa.c
$ node test.js
[ { moduleName: 'coffeescript/register', module: {}, error: null } ]
{}
[ { moduleName: 'coffeescript/register', module: {}, error: null } ]
{}
/path/to/rechoir/bug/node_modules/rechoir/index.js:30                                                                                    
    throw new Error('No module loader found for "' + usedExtension + '".');
    ^

Error: No module loader found for "undefined".
    at Object.exports.prepare (/path/to/rechoir/bug/node_modules/rechoir/index.js:30:11)
    at Object.<anonymous> (/path/to/rechoir/bug/test.js:14:21)
    at Module._compile (node:internal/modules/cjs/loader:1108:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1137:10)
    at Module.load (node:internal/modules/cjs/loader:973:32)
    at Function.Module._load (node:internal/modules/cjs/loader:813:14)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:76:12)
    at node:internal/main/run_main_module:17:47

Please provide the following information:

• OS & version [e.g. MacOS Catalina 10.15.4]: macOS Big Sur 11.2.1
• node version (run node -v): v15.5.0
• npm version (run npm -v): 7.3.0
• gulp version (run gulp -v): rechoir version: 0.7.0

Additional information

Nothing.