Fix handling of data_overrides in project factory

[wiktorn]

coalesce([], ["a"]) == [] and coalesce({}, {a = 1}) == {}, so it's wrong default value for data_overrides. Changed that to null + added few comments and tests.

Allow multiple project files of the same name in factory, as long as they override the name. If duplicate is found, following error is raised:

Error: Duplicate object key

  on factory-projects.tf line 39, in locals:
  35:   _projects_input = {
  36:     # will raise error, if the same filename is raised multiple times
  37:     # and project name is not set via name in YAML
  38:     for k, v in merge(local._hierarchy_projects_full_path, local._projects_full_path) :
  39:     lookup(v, "name", basename(k)) => v
  40:   }

Two different items produced the key "automation" in this 'for' expression.
If duplicates are expected, use the ellipsis (...) after the value expression
to enable grouping by key.

Which hopefully will help users to understand where the error is.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass