fix(eslint-plugin): [no-unnecessary-type-assertion] handle missing declarations

[bradzacher]

Fixes #532

The example in the issue works fine as long as all the required declarations are attached.
If something is missing, then it breaks because the code was assuming valid typescript.

[codecov]

Codecov Report

Merging #537 into master will decrease coverage by 0.03%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #537      +/-   ##
==========================================
- Coverage   94.15%   94.12%   -0.04%     
==========================================
  Files         104      104              
  Lines        4263     4271       +8     
  Branches     1163     1166       +3     
==========================================
+ Hits         4014     4020       +6     
- Misses        145      146       +1     
- Partials      104      105       +1

Impacted Files	Coverage Δ
...-plugin/src/rules/no-unnecessary-type-assertion.ts	90.78% <100%> (+0.24%)	⬆️
packages/eslint-plugin/src/util/types.ts	78.78% <71.42%> (-1.22%)	⬇️