efi/libstub: fix efi_parse_options() ignoring the default command line

efi_convert_cmdline() always returns a size of at least 1 because it
counts the NUL terminator, so the "cmdline_size == 0" condition is never
satisfied.

Change it to check if the string starts with a NUL character to get the
intended behavior: to use CONFIG_CMDLINE when load_options_size == 0.

Fixes: 60f38de ("efi/libstub: Unify command line param parsing")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/efi-stub.c

@@ -129,7 +129,7 @@ efi_status_t efi_handle_cmdline(efi_loaded_image_t *image, char **cmdline_ptr)
 
 	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) ||
 	    IS_ENABLED(CONFIG_CMDLINE_FORCE) ||
-	    cmdline_size == 0) {
+	    cmdline[0] == 0) {
 		status = efi_parse_options(CONFIG_CMDLINE);
 		if (status != EFI_SUCCESS) {
 			efi_err("Failed to parse options\n");