Allow writing partial SPDX documents without files

[pombredanne]

This is a follow up on #1011
This needs to be solved upstream in the SPDX python library: a document without files should be possible

[kestewart]

Method is to set "FilesAnalyzed" to false for the Package under review.
see: https://spdx.org/spdx-specification-21-web-version#h.ve38itss85w

[pombredanne]

@kestewart Thank you! This (FilesAnalyzed) is a part that is not yet supported in the SPDX python tools library.