Add support for reading Null Shapes

[kd35a]

Content of the test_null shapefile:

wkt_geom	NAME (10)
Point (3 3)	has geom
NULL	null geom

[codecov-io]

Codecov Report

Merging #225 into master will decrease coverage by 0.14%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   90.16%   90.01%   -0.15%     
==========================================
  Files          52       54       +2     
  Lines        1789     1803      +14     
  Branches      124      123       -1     
==========================================
+ Hits         1613     1623      +10     
- Misses        176      180       +4

Impacted Files	Coverage Δ
src/main/scala/magellan/Shape.scala	66.66% <ø> (+8.13%)	⬆️
src/main/scala/magellan/io/ShapeReader.scala	92.3% <100%> (+1.92%)	⬆️
src/main/scala/magellan/NullShape.scala	37.5% <37.5%> (ø)
...cala/org/apache/spark/sql/types/NullShapeUDT.scala	54.54% <54.54%> (ø)

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