fix: crash due to config packages key with missing value

- Treat empty 'packages:' entry as empty array to prevent NPE.
  Ensures consistent behavior with when no 'package:' entry or
  'package: []' is specified.
- Log and return null instead of System.exit(1) on 'load',
  since System.exit(1) is already done in 'initialize'

Added tests to verify:
- Loading config with 'packages' key having no value.
- Error message for scalar 'packages' value.

Author: edisigci

agent/src/main/java/com/appland/appmap/config/AppMapConfig.java

@@ -131,9 +131,13 @@ static AppMapConfig load(Path configFile, boolean mustExist) {
     ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
     try {
       singleton = mapper.readValue(inputStream, AppMapConfig.class);
+      if (singleton.packages == null) {
+        logger.error("AppMap: missing value for the 'packages' entry in appmap.yml");
+        return null;
+      }
     } catch (IOException e) {
       logger.error("AppMap: encountered syntax error in appmap.yml {}", e.getMessage());
-      System.exit(1);
+      return null;
     }
     singleton.configFile = configFile;
     logger.debug("config: {}", singleton);

agent/src/test/java/com/appland/appmap/config/AppMapConfigTest.java

@@ -100,6 +100,24 @@ public void hasAppmapDir() throws Exception {
         AppMapConfig.load(configFile, true);
         assertEquals("/appmap", AppMapConfig.get().getAppmapDir());
     }
+
+    @Test
+    public void loadPackagesKeyWithMissingValue() throws Exception {
+        Path configFile = tmpdir.resolve("appmap.yml");
+        final String contents = "name: test\npackages:\npath: xyz";
+        Files.write(configFile, contents.getBytes());
+        String actualErr = tapSystemErr(() -> AppMapConfig.load(configFile, false));
+        assertTrue(actualErr.contains("AppMap: missing value for the 'packages'"));
+    }
+
+    @Test
+    public void loadPackagesKeyWithScalarValue() throws Exception {
+        Path configFile = Files.createTempFile("appmap", ".yml");
+        final String contents = "name:q test\npackages: abc\n";
+        Files.write(configFile, contents.getBytes());
+        String actualErr = tapSystemErr(() -> AppMapConfig.load(configFile, false));
+        assertTrue(actualErr.contains("AppMap: encountered syntax error in appmap.yml"));
+    }
 }