GNSS Model Name constant string and comment updates

Addressing API review feedback
Removed string, returning Null instead
Updated comments

Bug: 73951626
Test: Runs on device w/GnssLogger
Change-Id: I54d05a7ae000e9ff52ec1f6646f83aed7de81b94
diff --git a/services/core/java/com/android/server/LocationManagerService.java b/services/core/java/com/android/server/LocationManagerService.java
index 65e90bad..19e5cc6 100644
--- a/services/core/java/com/android/server/LocationManagerService.java
+++ b/services/core/java/com/android/server/LocationManagerService.java
@@ -19,6 +19,7 @@
 import static android.content.pm.PackageManager.PERMISSION_GRANTED;
 
 import android.annotation.NonNull;
+import android.annotation.Nullable;
 import android.app.ActivityManager;
 import android.app.AppOpsManager;
 import android.app.PendingIntent;
@@ -1161,11 +1162,12 @@
      * Returns the model name of the GNSS hardware.
      */
     @Override
+    @Nullable
     public String getGnssHardwareModelName() {
         if (mGnssSystemInfoProvider != null) {
             return mGnssSystemInfoProvider.getGnssHardwareModelName();
         } else {
-            return LocationManager.GNSS_HARDWARE_MODEL_NAME_UNKNOWN;
+            return null;
         }
     }