fix(PolylinePoint): prevent accessing undefined variable on change

[mathstlouis]

Bug already discussed in the following issue:
#1268

When updating only the latitude or longitude of a PolylinePoint, an error is thrown by ngOnChanges.

[codecov]

Codecov Report

Merging #1597 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1597      +/-   ##
==========================================
+ Coverage   33.09%   33.35%   +0.26%     
==========================================
  Files          37       37              
  Lines        1644     1646       +2     
  Branches      129      131       +2     
==========================================
+ Hits          544      549       +5     
+ Misses       1098     1095       -3     
  Partials        2        2

Impacted Files	Coverage Δ
packages/core/directives/polyline-point.ts	100% <100%> (+30%)	⬆️

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[stale]

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