Output `null` when tests don't have a name

[SamVerschueren]

I just noticed this by accident when testing mem-map and couldn't find a similar issue.

AVA version: v0.11.0

The reason seems to be that I don't have a name for my test.

[jamestalmage]

Should be [anonymous]

[jamestalmage]

Actually - if there's just one test - it should be nothing.

[sotojuan]

This is the offending line. Should it just return an empty string?

[jamestalmage]

Yep. Empty string