fix: maps utils invariants for collections containing undefined values

[wschurman]

Why

A map can have undefined values on purpose, which makes it hard to distinguish from an undefined result of Map.get (could do a Map.has and invariant on that, but it's unnecessary in these cases since these invariants were just for programmer confidence, not API correctness or soundness).

How

Remove invariants.

Test Plan

Run tests (which test these extensively).

[codecov]

Codecov Report

Merging #103 (ba822c5) into master (15546aa) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   94.95%   94.95%   -0.01%     
==========================================
  Files          66       66              
  Lines        1626     1624       -2     
  Branches      196      195       -1     
==========================================
- Hits         1544     1542       -2     
  Misses         80       80              
  Partials        2        2              

Flag	Coverage Δ
integration	94.95% <100.00%> (-0.01%)	⬇️
unittest	94.95% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
packages/entity/src/utils/collections/maps.ts	100.00% <100.00%> (ø)

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