fix error due to adding null callback

[chriscoey]

replaces #447 (CI would not run on PR from fork)
fixes #395

[odow]

@simonbowly any chance you could take a look at our license?

I tried generating a new API key, but it's still tied to the expired license.

[simonbowly]

@odow sorry about that, I just updated the license. Should be valid now but you might need to generate the API key again.

I'll follow up to see why the duration is so short.

[odow]

That seems to have fixed it.

I'll follow up to see why the duration is so short.

Yes. Having a 2 week expiry isn't ideal...

[odow]

@chriscoey I excluded some new tests from MOI about infeasibility certificates which you can ignore.

But the heuristic callback test failing seems related?

[chriscoey]

the heuristic callback test failing seems unrelated to this PR, but I pushed a fix for it here

[codecov]

Codecov Report

Merging #448 (16dcb75) into master (1008e8c) will increase coverage by 1.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   54.57%   56.07%   +1.50%     
==========================================
  Files           9        9              
  Lines        3782     3916     +134     
==========================================
+ Hits         2064     2196     +132     
- Misses       1718     1720       +2     

Impacted Files	Coverage Δ
src/MOI_wrapper/MOI_wrapper.jl	88.50% <100.00%> (+0.27%)	⬆️
src/MOI_wrapper/MOI_callbacks.jl	97.61% <0.00%> (+0.14%)	⬆️
src/MOI_wrapper/MOI_indicator_constraint.jl	95.69% <0.00%> (+0.24%)	⬆️
src/gen95/libgrb_api.jl	25.52% <0.00%> (+8.73%)	⬆️

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