fix: source_map_full output in vyper-json

[charles-cooper]

source_map_full output fails with a KeyError when user requests evm.sourceMapFull but not evm.sourceMap. this commit fixes

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Codecov Report

Merging #3464 (4af66e8) into master (070b0cf) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3464      +/-   ##
==========================================
- Coverage   89.30%   89.27%   -0.04%     
==========================================
  Files          84       84              
  Lines       10794    10794              
  Branches     2462     2462              
==========================================
- Hits         9640     9636       -4     
- Misses        756      760       +4     
  Partials      398      398              

Impacted Files	Coverage Δ
vyper/cli/vyper_json.py	78.62% <100.00%> (ø)

... and 1 file with indirect coverage changes

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