Handle null PATH environment variable in UserAgent

System.getenv("PATH") returns null in environments where PATH is not
set (serverless runtimes, minimal containers). This caused an NPE in
UserAgent.env(). Fall back to an empty array when PATH is null.

Fixes #550

Author: renaudhartert-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/UserAgent.java

@@ -299,11 +299,9 @@ private static String agentProvider() {
 
   private static Environment env() {
     if (env == null) {
-      env =
-          new Environment(
-              System.getenv(),
-              System.getenv("PATH").split(File.pathSeparator),
-              System.getProperty("os.name"));
+      String pathEnv = System.getenv("PATH");
+      String[] pathEntries = pathEnv != null ? pathEnv.split(File.pathSeparator) : new String[0];
+      env = new Environment(System.getenv(), pathEntries, System.getProperty("os.name"));
     }
     return env;
   }

databricks-sdk-java/src/test/java/com/databricks/sdk/core/UserAgentTest.java

@@ -247,6 +247,16 @@ public void testAgentProviderEmptyValue() {
     Assertions.assertFalse(UserAgent.asString().contains("agent/"));
   }
 
+  @Test
+  public void testEnvWithNullPath() {
+    UserAgent.env =
+        new Environment(new HashMap<>(), new ArrayList<>(), System.getProperty("os.name"));
+    UserAgent.cicdProvider = null;
+    UserAgent.agentProvider = null;
+    String userAgent = UserAgent.asString();
+    Assertions.assertTrue(userAgent.contains("databricks-sdk-java/"));
+  }
+
   @Test
   public void testAgentProviderCached() {
     // Set up with cursor agent