fix(maven): validate lock file usage when no packages or requirements.txt are configured

Author: ih0r-d

graalpy-maven-plugin/src/main/java/org/graalvm/python/maven/plugin/InstallPackagesMojo.java

@@ -40,6 +40,7 @@
  */
 package org.graalvm.python.maven.plugin;
 
+import java.nio.file.Files;
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.LifecyclePhase;
 import org.apache.maven.plugins.annotations.Mojo;
@@ -100,7 +101,16 @@ private void manageVenv() throws MojoExecutionException {
 		MavenDelegateLog log = new MavenDelegateLog(getLog());
 		Path lockFile = getLockFile();
 		Path reqFile = resolveReqFile();
-		if ((packages == null || packages.isEmpty()) && reqFile == null) {
+
+		boolean emptyPackages = packages == null || packages.isEmpty();
+		boolean hasReqFile = reqFile != null && Files.exists(reqFile);
+		boolean hasLockFile = lockFile != null && Files.exists(lockFile);
+
+		if (emptyPackages && !hasReqFile) {
+			if (hasLockFile) {
+				throw new MojoExecutionException(
+						"Lock file is present, but no Python packages or requirements.txt are configured.");
+			}
 			return;
 		}
 		try {