fix(spdx): Handle missing linkage types for orphaned graph fragments

Use safe lookup for dependency edges that may not be in the linkage map.

Signed-off-by: Kai Hodžić <hodzic.e.k@outlook.com>

Author: voidpetal

plugins/reporters/spdx/src/main/kotlin/SpdxDocumentModelMapper.kt

@@ -71,10 +71,10 @@ internal object SpdxDocumentModelMapper {
                 relatedSpdxElement = toOrtId.toSpdxId()
             )
 
-            linkageTypesForDependencyRelationships.getValue(fromOrtId to toOrtId)
-                .mapTo(mutableSetOf()) { it.toSpdxRelationshipType() }
-                .sorted()
-                .forEach { relationshipType ->
+            linkageTypesForDependencyRelationships[fromOrtId to toOrtId]
+                ?.mapTo(mutableSetOf()) { it.toSpdxRelationshipType() }
+                ?.sorted()
+                ?.forEach { relationshipType ->
                     relationships += SpdxRelationship(
                         spdxElementId = fromSpdxId,
                         relationshipType = relationshipType,