Fix panic on empty sbom (#917)

* Implement fmt.Stringer with format.ID

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Add failing test for formats processing empty SBOMs

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Account for nil SPDX document during Syft model conversion

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

internal/formats/common/spdxhelpers/to_syft_model.go

@@ -1,6 +1,7 @@
 package spdxhelpers
 
 import (
+	"errors"
 	"strconv"
 	"strings"
 
@@ -17,6 +18,10 @@ import (
 )
 
 func ToSyftModel(doc *spdx.Document2_2) (*sbom.SBOM, error) {
+	if doc == nil {
+		return nil, errors.New("cannot convert SPDX document to Syft model because document is nil")
+	}
+
 	spdxIDMap := make(map[string]interface{})
 
 	s := &sbom.SBOM{