fix(ci): handle null user.home defensively in BomContentTest

Author: lqiu96

java-cloud-bom/tests/src/test/java/com/google/cloud/BomContentTest.java

@@ -154,8 +154,12 @@ private static String buildMavenCentralUrl(Artifact artifact) {
   private static boolean existsLocally(Artifact artifact) {
     String localRepository = System.getProperty("maven.repo.local");
     if (localRepository == null) {
+      String userHome = System.getProperty("user.home");
+      if (userHome == null) {
+        return false;
+      }
       // Standard default location for Maven local repository.
-      localRepository = System.getProperty("user.home") + "/.m2/repository";
+      localRepository = userHome + "/.m2/repository";
     }
     Path localPath =
         Paths.get(