[MOF] fix writing NLPBlock

[odow]

Two bugs:

1. We were still assuming NLPBlock used x[1] instead of x[VariableIndex(1)]
2. NLP objectives were being overwritten by empty linear/quadratic ones because the JSON file for the test was wrong :(

[codecov-io]

Codecov Report

Merging #1037 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
- Coverage   95.19%   95.19%   -0.01%     
==========================================
  Files         100      100              
  Lines       11344    11343       -1     
==========================================
- Hits        10799    10798       -1     
  Misses        545      545

Impacted Files	Coverage Δ
src/FileFormats/MOF/nonlinear.jl	100% <100%> (ø)	⬆️
src/FileFormats/MOF/write.jl	66.31% <100%> (+0.72%)	⬆️

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[odow]

Codecov is annoying again.

@blegat objections to merging this and then tagging 0.9.12?