fix: update license content filtering default case to be 'none' for no content returned

---------
Signed-off-by: Christopher Phillips <32073428+spiffcs@users.noreply.github.com>

syft/pkg/license.go

@@ -365,7 +365,7 @@ func (b *licenseBuilder) licensesFromEvidenceAndContent(evidence []licenses.Evid
 
 func (b *licenseBuilder) licenseFromContentHash(content string) License {
 	hash := sha256HexFromString(content)
-	value := "LicenseRef-sha256:" + hash
+	value := "sha256:" + hash
 
 	return License{
 		Value:     value,

syft/pkg/license_set_test.go

@@ -149,7 +149,7 @@ func TestLicenseSet_Add(t *testing.T) {
 					Locations:      file.NewLocationSet(),
 				},
 				{
-					Value:    "LicenseRef-sha256:eebcea3ab1d1a28e671de90119ffcfb35fe86951e4af1b17af52b7a82fcf7d0a",
+					Value:    "sha256:eebcea3ab1d1a28e671de90119ffcfb35fe86951e4af1b17af52b7a82fcf7d0a",
 					Contents: readFileAsString("../../internal/licenses/test-fixtures/nvidia-software-and-cuda-supplement"),
 					Type:     license.Declared,
 				},

syft/pkg/license_test.go

@@ -255,7 +255,7 @@ func TestFullText(t *testing.T) {
 			name:  "Full Text field is populated with the correct full text and contents are given a sha256 as value",
 			value: fullText,
 			want: License{
-				Value:    "LicenseRef-sha256:108067fa71229a2b98b9696af0ce21cd11d9639634c8bc94bda70ebedf291e5a",
+				Value:    "sha256:108067fa71229a2b98b9696af0ce21cd11d9639634c8bc94bda70ebedf291e5a",
 				Type:     license.Declared,
 				Contents: fullText,
 			},