Do not use empty filepath as event config

Summary:
Return early if the given filepath is empty

This is needed since the default MITK interaction event config
inside the mitkTool was changed from 'DisplayConfigMITK.xml' to
an empty string.
The reason was that in while segmenting in the PACS mode after a tool
has been disabled again the default config was loaded, which was MITK mode.

Test Plan:
manual testing

• Open MITK workbench
• load an image
• open a segmentation view
• change to 3D segmentations
• activate a tool
• observe output: no xml parsing error should be visible

Reviewers: kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Subscribers: s434n, kalali, kislinsk

Maniphest Tasks: T28667

Differential Revision: https://phabricator.mitk.org/D557