[algorithms][is_valid] accept a multi-linestring with no elements as valid
[algorithms][is_valid] accept a multi-polygon with no elements as valid
[algorithms][is_valid] accept a multi-point with no elements as valid
[doc][algorithms][is_valid] update doc to reflect that empty multi-geometries
are considered as valid
[test][algorithms][is_valid] modify unit test to accept multi-geometries
with no elements as valid
Merge pull request #184 from mkaravel/fix/validity_of_empty_multigeometries

Fix/validity of empty multigeometries