add hide-empty-text option to hide module whenever output is empty but format is not

[RobertMueller2]

Resolves #3341

Custom module hides empty output, e.g. when a worker returns {} as json. But when using other content in the format, e.g. to add an icon before or after output, it's no longer possible to have the module hidden.

This PR addresses this by adding a config option hide-empty-text, when true, text_ is checked for empty rather than str.

I realise I forgot the manpage, coming up in a second :P

[vikanezrimaya]

Am the original reporter, tested this on my install, can confirm this resolves my issue.

[Alexays]

LGTM, thx