fix issue 356 that ref to wrong type of map for nil value

[tiltwind]

What this PR does:

fix issue #356 that ref to wrong type of map for nil value

Which issue(s) this PR fixes:
Fixes #356

Special notes for your reviewer:
None

Does this PR introduce a user-facing change?:

fix issue #356 that ref to wrong type of map for nil value

[codecov-commenter]

Codecov Report

Merging #357 (f7ab7bd) into master (76ae75b) will decrease coverage by 0.03%.
The diff coverage is 90.00%.

❗ Current head f7ab7bd differs from pull request most recent head d1c8ccc. Consider uploading reports for the commit d1c8ccc to get more accurate results

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   72.18%   72.16%   -0.03%     
==========================================
  Files          27       27              
  Lines        3128     3129       +1     
==========================================
  Hits         2258     2258              
- Misses        633      634       +1     
  Partials      237      237              

Impacted Files	Coverage Δ
ref.go	83.11% <88.88%> (-1.10%)	⬇️
map.go	59.58% <100.00%> (ø)

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[wenxuwan]

lgtm