do not output a js file if there is no js block

[pietroppeter]

we talked about it in another conversation (that I cannot find at the moment), this is related but different from #164 and I wanted it track somewhere. cc @HugoGranstrom

the issue is that currently in nimib, even if we do not have nbJs blocks, at nbSave a out.js file is generated (compiling an empty nim file). we should skip this step of compiling an empty nim file. I thinks this is fixed in #163 (in can see the fix in jsutils.nim)