Missing icon in WID cause that task is not shown in Designer

Description of problem:
If a definition of WID contain empty the "icon" parameter then task won't be presented in "Service Task" section in the object Library.

Version-Release number of selected component (if applicable):

Steps to Reproduce:
1. create new WID
2. delete the value of "icon" parameter ("icon" : "")
3. validate it and save it
4. create new process in the same package

Actual results:
Object Library does not contains this custom task. Validation (step 3) was successful.

Expected results:
Use the default icon in Designer. The guided decision table editor does not require an icon in WID.