fix: fd-tree throwing error when used without header

[migg24]

Please provide a link to the associated issue.

No issue, found the issue while using fd-tree.

Please provide a brief summary of this pull request.

When using the <fd-tree> without a [header] this part of the code throws errors because this.header is undefined, practically making [header] a mandatory attribute even if you do not need headers (currently you need to provide an empty array).

If this is a new feature, have you updated the documentation?

As a side note the documentation for I could not find on https://sap.github.io/fundamental-ngx/home.

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[mikerodonnell89]

While this is currently a prototype, undocumented component at the moment, we're liking this PR. Thanks @migg24