Fix loading multi-doc YAMLs for CI configurations

What does this MR do and why?

Related to Multi document YAML doesn't work with leading a... (#396846 - closed)

In this MR we fixes 2 bugs:

Fix 1

job:
  script: echo hello world
--- # start of the 2nd document
something: ignored

If the YAML contains multiple documents, before the multi-doc YAML we were only loading the first document. After multi-doc YAML changes we started loading multiple documents and assumed that the 1st document, when multiple docs present in the file, is always the config Header defining spec:inputs:.

The error users were seeing errors such Included file <PATH/TO/YML> does not have valid YAML syntax! because we were considering the 2nd document as the config content.

Fix 2

job:
  script: echo hello world
--- # start of the 2nd document

After fixing the 1st bug, when the 2nd document is empty we started raising a NotHashError. We needed a way to reject empty documents in order to maintain backwards compatibility.

Since whether to reject documents is not the Loader::MultiDocYaml responsibility, we now pass a reject_empty: true option from CI code (Gitlab::Ci::Config::Yaml).

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