Change DualObjectiveValue to use Utilities.get_fallback

[odow]

@blegat: I'm not really sure what we originally intended here.

help?> MOI.DualObjectiveValue
  DualObjectiveValue(result_index::Int = 1)

  A model attribute for the value of the objective function of the dual problem for the result_indexth dual result.

  If the solver does not have a dual value for the objective because the result_index is beyond the available solutions (whose number is indicated by the ResultCount attribute), getting this attribute
  must throw a ResultIndexBoundsError. Otherwise, if the result is unavailable for another reason (for instance, only a primal solution is available), the result is undefined. Users should first check
  DualStatus before accessing the DualObjectiveValue attribute.

  See ResultCount for information on how the results are ordered.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.87%. Comparing base (6e12c8a) to head (757559c).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #626      +/-   ##
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+ Coverage   93.55%   93.87%   +0.32%     
==========================================
  Files           6        6              
  Lines        2715     2710       -5     
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+ Hits         2540     2544       +4     
+ Misses        175      166       -9     

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[blegat]

It should be the dual objective evaluated at the dual solution so it's indeed not the objective bound of MIPs