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: 60f38de7 ("efi/libstub: Unify command line param parsing")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>