Fix magnitude warning with zero coefficients

[odow]

Avoids this stuff

       27L   9.467399e+05   2.713322e+04   3.014880e+01          1       6480
       28L   7.112808e+05   2.713324e+04   3.205402e+01          1       6720
┌ Warning: Found a cut with a mix of small and large coefficients.
│     The order of magnitude difference is 13.898108414238555.
│     The smallest cofficient is -1.264420666934206e-14.
│     The largest coefficient is 0.0.
│ 
│ You can ignore this warning, but it may be an indication of numerical issues.
│ 
│ Consider rescaling your model by using different units, e.g, kilometers instead
│ of meters. You should also consider reducing the accuracy of your input data (if
│ you haven't already). For example, it probably doesn't make sense to measure the
│ inflow into a reservoir to 10 decimal places.
└ @ SDDP ~/.julia/dev/SDDP/src/plugins/bellman_functions.jl:68