Improves spdx.package validations: Now validates none str optional and mandatory fields. Two helper functions added validate_optional_fields and validate_mandatory_fields.
Improves spdx.package validations:
Now validates none str optional and mandatory fields.
Two helper functions added validate_optional_fields and validate_mandatory_fields.

Signed-off-by: Ahmed H. Ismail <17be99835f39b7679f83bf2695d44e1f53f60100@gmail.com>

spdx/tools-python,ah450/spdx-tools-python,bmwcarit/spdx-tools-python