fix: avoid NPE when requirements.txt is not used

Author: ih0r-d

org.graalvm.python.embedding.tools/src/main/java/org/graalvm/python/embedding/tools/vfs/VFSUtils.java

@@ -40,6 +40,7 @@
  */
 package org.graalvm.python.embedding.tools.vfs;
 
+import VFSUtils.LockFile;
 import java.io.Serial;
 import org.graalvm.python.embedding.tools.exec.BuildToolLog;
 import org.graalvm.python.embedding.tools.exec.BuildToolLog.CollectOutputLog;
@@ -552,24 +553,24 @@ public static void createVenv(Path venvDirectory, List<String> packages, Path lo
 
 	static boolean installPackagesFromReqFile(Path venvDirectory, Launcher launcher, String graalPyVersion,
 			BuildToolLog log, Path lockFilePath, Path reqFile) throws IOException {
-		if (reqFile != null && Files.exists(reqFile)) {
-			log.info("Using <requirements.txt> dependency mode.");
-			log.info("Installing Python dependencies from: " + reqFile);
+		if (reqFile == null || !Files.exists(reqFile)) {
+			return false;
+		}
+		log.info("Using <requirements.txt> dependency mode.");
+		log.info("Installing Python dependencies from: " + reqFile);
 
-			warnIfLockFileExists(lockFilePath, log);
+		warnIfLockFileExists(lockFilePath, log);
 
-			VenvContents vc = ensureVenv(venvDirectory, graalPyVersion, launcher, log);
+		VenvContents vc = ensureVenv(venvDirectory, graalPyVersion, launcher, log);
 
-			runPip(venvDirectory, "install", log, "--compile", "-r", reqFile.toString());
-			List<String> reqPackages = requirementsPackages(reqFile);
-			vc.write(reqPackages);
+		runPip(venvDirectory, "install", log, "--compile", "-r", reqFile.toString());
+		List<String> reqPackages = requirementsPackages(reqFile);
+		vc.write(reqPackages);
 
-			InstalledPackages installed = InstalledPackages.fromVenv(venvDirectory);
-			installed.freeze(log);
+		InstalledPackages installed = InstalledPackages.fromVenv(venvDirectory);
+		installed.freeze(log);
 
-			return true;
-		}
-		return false;
+		return true;
 	}
 
 	private static void warnIfLockFileExists(Path lockFilePath, BuildToolLog log) {
@@ -591,7 +592,7 @@ static void installPackages(Path venvDirectory, List<String> packages, Path lock
 
 		List<String> pluginPackages = trim(packages);
 
-		LockFile lockFile = null;
+		VFSUtils.LockFile lockFile = null;
 		if (lockFilePath != null && Files.exists(lockFilePath)) {
 			log.info("Lock-file detected: " + lockFilePath);
 			lockFile = LockFile.fromFile(lockFilePath, log);