Return null when getting role holders of an unknown role.

Previously the code was accidentally relying on ArraySet constructor
accepting a null argument when the role is not found, so make it
return null in this case to match both the javadoc and what we
intended.

Bug: 110557011
Test: build
Change-Id: Ifef270a84854c105d9627ee15e96a649dfac0abf
diff --git a/services/core/java/com/android/server/role/RoleUserState.java b/services/core/java/com/android/server/role/RoleUserState.java
index 69e1449..02dcc49 100644
--- a/services/core/java/com/android/server/role/RoleUserState.java
+++ b/services/core/java/com/android/server/role/RoleUserState.java
@@ -193,14 +193,18 @@
      *
      * @param roleName the name of the role to query for
      *
-     * @return the set of role holders. {@code null} should not be returned and indicates an issue.
+     * @return the set of role holders, or {@code null} if and only if the role is not found
      */
     @Nullable
     public ArraySet<String> getRoleHolders(@NonNull String roleName) {
         synchronized (mLock) {
             throwIfDestroyedLocked();
 
-            return new ArraySet<>(mRoles.get(roleName));
+            ArraySet<String> packageNames = mRoles.get(roleName);
+            if (packageNames == null) {
+                return null;
+            }
+            return new ArraySet<>(packageNames);
         }
     }
 
@@ -268,8 +272,7 @@
      * @param roleName the name of the role to add the holder to
      * @param packageName the package name of the new holder
      *
-     * @return {@code false} only if the set of role holders is null, which should not happen and
-     *         indicates an issue.
+     * @return {@code false} if and only if the role is not found
      */
     @CheckResult
     public boolean addRoleHolder(@NonNull String roleName, @NonNull String packageName) {
@@ -302,8 +305,7 @@
      * @param roleName the name of the role to remove the holder from
      * @param packageName the package name of the holder to remove
      *
-     * @return {@code false} only if the set of role holders is null, which should not happen and
-     *         indicates an issue.
+     * @return {@code false} if and only if the role is not found
      */
     @CheckResult
     public boolean removeRoleHolder(@NonNull String roleName, @NonNull String packageName) {