Fixes #28637 - set minimal headers for empty reports

When the report does not render any row, resulting file does not have
any headers. This can be confusing for users, so render_headers macro
was added. It should specify column names that user should always see.
In case some columns are added dynamically, and report_row adds some
additional columns, they are added on demand. Meaning, if we iterate
over some data, new columns can be still added.