kconfig: default to zero if int/hex symbol lacks default property

When a default property is missing in an int or hex symbol, it defaults
to an empty string, which is not a valid symbol value.

It results in an incorrect .config, and can also lead to an infinite
loop in scripting.

Use "0" for int and "0x0" for hex as a default value.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Yoann Congal <yoann.congal@smile.fr>