Add string fallback to README examples (#108)

Author: ucsurf

README.md

@@ -41,15 +41,15 @@ Pass a Basic auth header to the `parse()` method. If parsing fails
 
 ```js
 const { parse } = require('basic-auth');
-const user = parse(req.headers.authorization);
+const user = parse(req.headers.authorization ?? '');
 // => { name: 'something', pass: 'whatever' }
 ```
 
 A header string from any other location can also be parsed for example a `Proxy-Authorization` header:
 
 ```js
 const { parse } = require('basic-auth');
-const user = parse(req.getHeader('Proxy-Authorization'));
+const user = parse(req.getHeader('Proxy-Authorization') ?? '');
 ```
 
 A credentials object can be formatted with `auth.format` as
@@ -71,7 +71,7 @@ const compare = require('tsscmp');
 
 // Create server
 const server = http.createServer(function (req, res) {
-  const credentials = parse(req.headers.authorization);
+  const credentials = parse(req.headers.authorization ?? '');
 
   // Check credentials
   // The "check" function will typically be against your user store