Write dimension when exporting EMPTY to WKT

[paleolimbot]

Fixes #141.

unclass(wk::as_wkt(wk::as_wkb("POLYGON ZM EMPTY")))
#> [1] "POLYGON ZM EMPTY"

^{Created on 2022-09-18 by the reprex package (v2.0.1)}

[codecov-commenter]

Codecov Report

Base: 98.95% // Head: 98.95% // No change to project coverage 👍

Coverage data is based on head (20db238) compared to base (bb18e2c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files          74       74           
  Lines        5172     5172           
=======================================
  Hits         5118     5118           
  Misses         54       54           

Impacted Files	Coverage Δ
src/wkt-writer.cpp	99.43% <100.00%> (ø)

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