Fix null example in report specs (#3336)

`example` is a list of strings, as such it cannot be left empty, `null`.
To reset the list to "no examples" state, use an empty list.

spec/plans/report.fmf

@@ -4,7 +4,7 @@ story:
     the testing is finished.
 description:
     Report test results according to user preferences.
-example:
+example: []
 
 /display:
     summary: Show results in the terminal window