fix: missing handler in server middleware

[tobiasdiez]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

In the context of building storybook in bridge, I got an empty server middleware object for some reason (not sure why). This then lead to _handler being undefined and thus app.use throws an error.
As a quick fix, I've added an if check to test if _handler is defined before calling app.use.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)