Do not save PipelineMetadata if name is blank

What does this MR do and why?

Describe in detail what your merge request does and why.

When the pipeline name is blank after substituting variables, if we try saving the name to database, it fails because of a model (Ci::PipelineMetadata) validation. And the failure to save PipelineMetadata prevents the pipeline from being created.

The most common way of using the pipeline name is expected to be the following:

workflow:
  name: '$PIPELINE_NAME'
  rules:
    - if: $VAR1
      variables:
        PIPELINE_NAME: 'Pipeline 1'

    - if: $VAR2
      variables:
        PIPELINE_NAME: 'Pipeline 2'

    - when: always

The above usage will result in the pipeline failing to be created:

• If we have not named every possible pipeline.
  We want customers to be able to incrementally increase their usage of this feature, rather than forcing all pipelines to be named, or none.

• If we have not set a default value for the PIPELINE_NAME variable.
  Setting a default value for PIPELINE_NAME is not desirable since it will result in pipelines having a generic name.

To make it possible to name only some pipelines, this MR does not save the pipeline name to database if the name is blank after substituting variables.

#372538 (closed)

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