Added empty volume for styling files, to prevent reference errors

[Daimanta]

Description

When a custom mapfile is chosen, the "styling-files" volume will not be created. Since this volume is mounted unconditionally, this produces an error. This PR adds an empty volume in case there is a mapfile, preventing the mounting issues.

Type of change

• Bugfix

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR