`workflow:name` does not accept null/empty string
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## Problem to solve
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The CI/CD YAML syntax reference states for the [`workflow:name`](https://docs.gitlab.com/ci/yaml/#workflowname) keyword (emphasis mine):
> **Additional details**:
>
> * If the name is an **empty string**, the pipeline is not assigned a name. A name consisting of only CI/CD variables could evaluate to an empty string if all the variables are also empty.
This is wrong, since [`workflow:name`](https://docs.gitlab.com/ci/yaml/#workflowname) doesn't accept an empty string and pipeline creation errors with

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The following minimal `.gitlab-ci.yml` file doesn't work:
```yaml
workflow:
name: ""
job:
script:
- echo "dummy job"
```
However, it seems that [`workflow:name`](https://docs.gitlab.com/ci/yaml/#workflowname) does accept the `null` type, which correctly triggers the described behavior:
```yaml
workflow:
name: null
job:
script:
- echo "dummy job"
```
## Proposal
<!--Further specifics for how can we solve the problem.-->
Replace _empty string_ in the mentioned paragraph with `null`.
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issue