In case of 0 buy or 0 sell, replace the NULL spread by the spread to marketprice

[ghost]

Atm, in case of 0 buy or 0 sell, the displayed spread in the offer book is "-"
which is of few information for the possible customers.

(This morning, there are 18 non-null spreads, and 17 "-" spreads in the offer book.)

If the marketprice is known in the code (?)
maybe the "-" spread could be replaced by the spread to the marketprice.
It would convey a bit more information.

I don't know enough from the inside code to know if this may be easy or not. I put it here in case it's an easy thing to do.

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