sanitycheck: actually allow empty west-flash without parameter

According to west-flash option description it's allowed to use
west-flash without parameter:
> There are three ways this option is used.
> 1) bare: --west-flash
> 2) with a value: --west-flash="--board-id=42"
> 3) Multiple values: --west-flash="--board-id=42,--erase"

However, we don't allow to west-flash to be without parameter
when we verify sanitycheck arguments. Fix that.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>