Fix exception when route doesn't match configured filter

[TomaszG]

Description

Check if the next function is defined/provided before executing it

Motivation and Context

Fixes #870

How has this been tested?

Verified whether the error is not thrown when middleware receives a route that doesn't match the filter.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.