Making last two testcases pass in scala-test-runner

[DanKH]

changing call to “toExercismJSON” to Nil instead of null and adding predicament on the check in case of an empty testResultsFiles list

[mk-mxp]

For reference:

Making last two testcases pass in case of an empty testResultsList

The OP is looking for reviewers.

[BNAndras]

CC @grzegorz-bielski