Make resolveExpressionNodeToType() handle null

[felixfbecker]

Fixes #252
@thelfensdrfer please try this out

[codecov]

Codecov Report

Merging #277 into master will increase coverage by -0.19%.

@@             Coverage Diff             @@
##             master    #277      +/-   ##
===========================================
- Coverage     86.29%   86.1%   -0.19%     
- Complexity      722     723       +1     
===========================================
  Files            54      54              
  Lines          1459    1461       +2     
===========================================
- Hits           1259    1258       -1     
- Misses          200     203       +3

Impacted Files	Coverage Δ	Complexity Δ
src/DefinitionResolver.php	82.05% <100%> (-0.59%)	276 <ø> (+1)

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[thelfensdrfer]

Thanks it worked: [Info - 12:59:30 PM] All 8083 PHP files parsed in 353 seconds. 262 MiB allocated.