Discussion on skipping examples

Continuing the discussion from PR #788 to double check some stuff. The contributing guide says
```
Filtering prompts. If a prompt is applied to an example and produces an empty string,
that prompt/example pair will be skipped. You can therefore create prompts that only
apply to a subset of the examples by wrapping them in Jinja if statements.
```
I could have sworn it was either a blank input/output/or totally blank, as supported by the way the tests are written. Did we change this at some point? Perhaps we can run a check on existing prompts or change the code further to allow all options?

Also related to #749.