Undefined Index - Security Risk vs Performance vs Code Bloat

I'm collaborating on a project where the REST APIs basically break in development mode because it has a more include error reporting policy. Here's a typical line in this project:

public function someAction() {
   // Returns a map of params => values sent with HTTP req
   $params = $this->getParams();

   // This key may not exist --+
   //                          |
   //                          v
   $someField = $params['someField'] ?: 'default value';
   $someField = $this->sanitizeInput($someField);

   // ...
}

As a result, the JSON response in dev mode will often be littered with PHP: Notice: Undefined Index warnings, which will break the JSON output string.

My questions

• What exactly is the security risk (if any) in assuming that a variable has been initialized, particularly when pulling it from $_GET or $_POST?
• Would it be worth the trouble to go through and wrap every access to some assumed array key with isset() or array_key_exists()?
  • I've added isset() around individual keys that raise undef index warnings under certain actions throughout the app, but the code looks super bloated now...