Fix compile error with IGNORE_MISC_WIPE_DATA in some trees

Change-Id: I4a57e4e90158d50a9e9c9aef8ed1da9ee9179554
diff --git a/bootloader_message_twrp/bootloader_message.cpp b/bootloader_message_twrp/bootloader_message.cpp
index a7a58f6..a06ad9a 100644
--- a/bootloader_message_twrp/bootloader_message.cpp
+++ b/bootloader_message_twrp/bootloader_message.cpp
@@ -332,7 +332,7 @@
 // walking through the factory reset screen - without actually doing a factory reset
 #ifdef IGNORE_MISC_WIPE_DATA
                 if (!strcmp(arg, "--wipe_data")) {
-                    (*argv)[*argc] = "";
+                    (*argv)[*argc] = NULL;
                     *argc = *argc -1;
                     printf("Bootloader arg \"%s\" ignored because TWRP was compiled with TW_IGNORE_MISC_WIPE_DATA\n", arg);
                     continue;