refactor(spdx): Move nullOrBlankToSpdxNoassertionOrNone()

Because probably being the most natural string representation for an
`SpdxExpression`, it makes sense to move that function to `spdx-utils`.

Note that in order to continue working for `null`, it needs to stay an
extension function and not just override `toString()` in `SpdxExpression`.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

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

@@ -55,6 +55,7 @@ import org.ossreviewtoolkit.utils.spdx.model.SpdxExtractedLicenseInfo
 import org.ossreviewtoolkit.utils.spdx.model.SpdxFile
 import org.ossreviewtoolkit.utils.spdx.model.SpdxPackage
 import org.ossreviewtoolkit.utils.spdx.model.SpdxPackageVerificationCode
+import org.ossreviewtoolkit.utils.spdx.nullOrBlankToSpdxNoassertionOrNone
 import org.ossreviewtoolkit.utils.spdx.toSpdx
 import org.ossreviewtoolkit.utils.spdx.toSpdxId
 
@@ -251,16 +252,6 @@ internal fun SpdxDocument.addExtractedLicenseInfo(licenseTextProvider: LicenseTe
     return copy(hasExtractedLicensingInfos = extractedLicenseInfo)
 }
 
-/**
- * Convert an [SpdxExpression] to `NOASSERTION` if null, to `NONE` if blank, or to its string representation otherwise.
- */
-private fun SpdxExpression?.nullOrBlankToSpdxNoassertionOrNone(): String =
-    when {
-        this == null -> SpdxConstants.NOASSERTION
-        toString().isBlank() -> SpdxConstants.NONE
-        else -> toString()
-    }
-
 /**
  * Convert a null or blank [String] to `NONE`.
  */

utils/spdx/src/main/kotlin/Extensions.kt

@@ -28,6 +28,16 @@ import org.ossreviewtoolkit.utils.spdx.SpdxExpression.Strictness
 
 private val logger = loggerOf(MethodHandles.lookup().lookupClass())
 
+/**
+ * Convert an [SpdxExpression] to `NOASSERTION` if null, to `NONE` if blank, or to its string representation otherwise.
+ */
+fun SpdxExpression?.nullOrBlankToSpdxNoassertionOrNone(): String =
+    when {
+        this == null -> SpdxConstants.NOASSERTION
+        toString().isBlank() -> SpdxConstants.NONE
+        else -> toString()
+    }
+
 /**
  * Create an [SpdxLicenseIdExpression] from this [SpdxLicense].
  */