suggestion: hide "nil" outputs

[AlbertLei]

At this time, the following input:

`foo = 42`{=run_lua}

The answer is: `foo`{=run_lua}

Will output:

nil

The answer is: 42

Maybe it makes sense to hide "nil"? It seems sufficient to insert the following line at Line 19 of the lua filter

if result == nil then return {} end

ps: This is a very inspiring filter for me. Thx!