fix: Handle empty 'exclude' field in appmap.yml

This commit resolves a NullPointerException that occurred when the
`exclude` field within a package definition in `appmap.yml` was
present but empty (e.g., `exclude:`). The Jackson YAML parser would
deserialize this as `null`, leading to a crash when its length was
accessed.

The fix involves two parts:
1.  Modifying `AppMapConfig.java` to add a null check in the stream
    reduction logic for `p.exclude`, treating a null array as having a
    length of zero.
2.  Initializing the `exclude` field as an empty array in the
    `AppMapPackage` constructor using `@JsonCreator`, ensuring it's
    never null during deserialization.

A new regression test case, `loadEmptyExcludeField`, has been added to
`AppMapConfigTest.java` to verify that an empty `exclude` field is now
handled correctly and does not cause a crash. The test ensures that
the `exclude` array is non-null and has a length of zero, confirming
the intended behavior.

Author: dividedmind

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

@@ -145,7 +145,7 @@ static AppMapConfig load(Path configFile, boolean mustExist) {
 
     int count = singleton.packages.length;
     count = Arrays.stream(singleton.packages).map(p -> p.exclude).reduce(count,
-        (acc, e) -> acc += e.length, Integer::sum);
+        (acc, e) -> acc += e == null ? 0 : e.length, Integer::sum);
 
     int pattern_threshold = Properties.PatternThreshold;
     if (count > pattern_threshold) {

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

@@ -21,6 +21,17 @@ public class AppMapPackage {
   public boolean shallow = false;
   public Boolean allMethods = true;
 
+  @JsonCreator
+  public AppMapPackage(@JsonProperty("path") String path,
+                         @JsonProperty("exclude") String[] exclude,
+                         @JsonProperty("shallow") Boolean shallow,
+                         @JsonProperty("allMethods") Boolean allMethods) {
+    this.path = path;
+    this.exclude = exclude == null ? new String[] {} : exclude;
+    this.shallow = shallow != null && shallow;
+    this.allMethods = allMethods == null ? true : allMethods;
+  }
+
   public static class LabelConfig {
 
     private Pattern className = null;

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

@@ -118,6 +118,19 @@ public void loadPackagesKeyWithScalarValue() throws Exception {
         String actualErr = tapSystemErr(() -> AppMapConfig.load(configFile, false));
         assertTrue(actualErr.contains("AppMap: encountered syntax error in appmap.yml"));
     }
-}
 
+    @Test
+    public void loadEmptyExcludeField() throws Exception {
+        Path configFile = tmpdir.resolve("appmap.yml");
+        final String contents = "name: test\npackages:\n- path: com.example\n  exclude:\n";
+        Files.write(configFile, contents.getBytes());
+        
+        AppMapConfig config = AppMapConfig.load(configFile, false);
+        assertNotNull(config);
+        assertEquals(1, config.packages.length);
+        assertEquals("com.example", config.packages[0].path);
+        assertNotNull(config.packages[0].exclude);
+        assertEquals(0, config.packages[0].exclude.length);
+    }
+}