Fix Encoded Polyline of EMPTY

[Komzpa]

Ticket: https://trac.osgeo.org/postgis/ticket/3982

[codecov]

Codecov Report

Merging #189 into svn-trunk will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##           svn-trunk     #189      +/-   ##
=============================================
+ Coverage      79.24%   79.24%   +<.01%     
=============================================
  Files            201      201              
  Lines          62894    62901       +7     
=============================================
+ Hits           49840    49848       +8     
+ Misses         13054    13053       -1

Impacted Files	Coverage Δ
liblwgeom/lwout_encoded_polyline.c	95.83% <100%> (+0.37%)	⬆️
liblwgeom/cunit/cu_out_encoded_polyline.c	96% <100%> (+0.54%)	⬆️
liblwgeom/lwline.c	88.97% <0%> (+0.39%)	⬆️

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