In my PR #307 I introduced an error:
aggregateLanguageSummary returned nil instead of an empty slice in case of an empty input.

Sorry, my inexperience in Go fired back.
I was not really aware of the differences between:

language := []LanguageSummary{}

and

var language = []LanguageSummary

and blindly trusted the coding recommendation of GoLand to prefer the non-allocating version.

This issue caused the unit test TestToJSONEmpty to fail which I missed amongst all the other test currently failing in master.