Add a generic error message for missing vendor error strings.

The message is empty, instead of null, to prevent apps from crashing but it will be changed to a more meaninful string in a future change.

Fix: 196176475
Test: N/A
Change-Id: I7823084238a87cb1b8892b8550c472cd8b40f556

core/java/android/hardware/face/FaceManager.java

@@ -790,6 +790,11 @@ public class FaceManager implements BiometricAuthenticator, BiometricFaceConstan
                 }
             }
         }
+
+        // This is used as a last resort in case a vendor string is missing
+        // It should not happen for anything other than FACE_ERROR_VENDOR, but
+        // warn and use the default if all else fails.
+        // TODO(b/196639965): update string
         Slog.w(TAG, "Invalid error message: " + errMsg + ", " + vendorCode);
         return "";
     }

core/java/android/hardware/fingerprint/FingerprintManager.java

@@ -1386,8 +1386,13 @@ public class FingerprintManager implements BiometricAuthenticator, BiometricFing
                 }
             }
         }
+
+        // This is used as a last resort in case a vendor string is missing
+        // It should not happen for anything other than FINGERPRINT_ERROR_VENDOR, but
+        // warn and use the default if all else fails.
+        // TODO(b/196639965): update string
         Slog.w(TAG, "Invalid error message: " + errMsg + ", " + vendorCode);
-        return null;
+        return "";
     }
 
     /**