Return empty string when invalid error codes are received

Bug: 134857990

Test: Builds
Change-Id: I2850007f5aa5a00fa3ef7d95138200a3e6cec559
diff --git a/core/java/android/hardware/face/FaceManager.java b/core/java/android/hardware/face/FaceManager.java
index 8596af10..c8f0e09 100644
--- a/core/java/android/hardware/face/FaceManager.java
+++ b/core/java/android/hardware/face/FaceManager.java
@@ -637,7 +637,7 @@
             }
         }
         Slog.w(TAG, "Invalid error message: " + errMsg + ", " + vendorCode);
-        return null;
+        return "";
     }
 
     /**