SPDX 2 to 3 conversion should not fail if primary_package_purpose is empty

Signed-off-by: Holger Frydrych <holger.frydrych@tngtech.com>

Author: fholger

src/spdx_tools/spdx3/bump_from_spdx2/package.py

@@ -83,7 +83,9 @@ def bump_package(
         elif isinstance(id_or_ref, ExternalIdentifier):
             external_identifier.append(id_or_ref)
 
-    package_purpose = SoftwarePurpose[spdx2_package.primary_package_purpose.name]
+    package_purpose = (
+        SoftwarePurpose[spdx2_package.primary_package_purpose.name] if spdx2_package.primary_package_purpose else None
+    )
 
     payload.add_element(
         Package(