Add support for Arch (which doesnt return a version) and Open Suse Leap

Signed-off-by: Samuel Dacanay <sam.dacanay@anchore.com>

remove bad-version test fixture (no longer relevant since it is now an expected case) and add cases for arch and opensuse-leap

Signed-off-by: Samuel Dacanay <sam.dacanay@anchore.com>

Reduce nesting, simplify empty version logic

Signed-off-by: Samuel Dacanay <sam.dacanay@anchore.com>

syft/distro/identify_test.go

@@ -65,6 +65,15 @@ func TestIdentifyDistro(t *testing.T) {
 			fixture: "test-fixtures/os/unmatchable",
 			Type:    UnknownDistroType,
 		},
+		{
+			fixture: "test-fixtures/os/opensuse-leap",
+			Type:    OpenSuseLeap,
+			Version: "15.2.0",
+		},
+		{
+			fixture: "test-fixtures/os/arch",
+			Type:    ArchLinux,
+		},
 	}
 
 	observedDistros := internal.NewStringSet()
@@ -94,6 +103,11 @@ func TestIdentifyDistro(t *testing.T) {
 				return
 			}
 
+			if d.Version == nil {
+				t.Log("Distro doesn't have a Version")
+				return
+			}
+
 			if d.Version.String() != test.Version {
 				t.Errorf("expected distro version doesn't match: %v != %v", d.Version.String(), test.Version)
 			}
@@ -175,10 +189,6 @@ func TestParseOsReleaseFailures(t *testing.T) {
 		fixture string
 		name    string
 	}{
-		{
-			fixture: "test-fixtures/bad-version",
-			name:    "No version",
-		},
 		{
 			fixture: "test-fixtures/bad-id",
 			name:    "No name ID",