CI: finetune pipelines after removing entry points
# Bugs
## Description
After removing the entry point in `index.html`, all stages of the current pipeline should be aborted.
### I expected the following to happen
After removing the entry point in `index.html`, all stages of the current pipeline should be aborted.
### On the contrary, I observed
Only the `build` stage is omitted, while the process still tries to create a page in the `pages` job. This results in an error because the site hasn't been built in the first place.
## How to reproduce the bug
_Steps:_
* create a branch that has an entry point set in `index.html`
* merge this branch to `develop`
* the `pages` job fails, but the pipeline is fixed by the next pipeline triggered by the automatic commit
## Severity
How much impact does this bug have on the product or further development?
* [x] Minor
## Related Tickets
Cf. #203
/cc [Mathias Göbel](https://gitlab.gwdg.de/mgoebel), [Frank Schneider](https://gitlab.gwdg.de/schneider210), [Michelle Weidling](https://gitlab.gwdg.de/mrodzis)
issue