Nits - APK installed optimizations (#227)

Hello,

ported to APK installed parser common optimizations already merged for
DPKG in #168.
Additionally, a couple of staticcheck linter errors have been corrected
(I think!).
I saw the comments here:
https://github.com/google/osv-scanner/blob/fb4d2c4d8e4e59961db70121e31870593e045a4e/pkg/osvscanner/osvscanner.go#L39-L43
but after my change both lints and tests are ok so it's not clear to me
if comments are now out of date or if I'm missing something!

Comments have been introduced in #149.

Thank you,
Regards.

pkg/lockfile/apk-installed.go

@@ -17,15 +17,14 @@ func groupApkPackageLines(scanner *bufio.Scanner) [][]string {
 	for scanner.Scan() {
 		line := scanner.Text()
 
-		if line == "" {
-			if len(group) > 0 {
-				groups = append(groups, group)
-			}
-			group = make([]string, 0)
-
+		if line != "" {
+			group = append(group, line)
 			continue
 		}
-		group = append(group, line)
+		if len(group) > 0 {
+			groups = append(groups, group)
+		}
+		group = make([]string, 0)
 	}
 
 	if len(group) > 0 {
@@ -56,7 +55,7 @@ func parseApkPackageGroup(group []string, pathToLockfile string) PackageDetails
 	if pkg.Version == "" {
 		pkgPrintName := pkg.Name
 		if pkgPrintName == "" {
-			pkgPrintName = "<unknown>"
+			pkgPrintName = unknownPkgName
 		}
 
 		_, _ = fmt.Fprintf(