Error when getDefaultFileExtensions returns null

Problem/Motivation

In using the External Media module, it has highlighted an error with the `getFileExtensions` method in the `WebformManagedFileBase` class which can do a `str_replace` on a `null` value because `getDefaultFileExtensions` can return `null`.

Steps to reproduce

1. Install and enable the Webform and External Media modules
2. Add an External Media File element to a webform
3. When on the build page for the webform, the error should appear

Proposed resolution

The getDefaultFileExtensions returns an empty string not `null`

Remaining tasks

User interface changes

API changes

Data model changes