Support 'POINT EMPTY' conversion to geo_types

[rmanoka]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

closes #61

[rmanoka]

failures:
    conversion::tests::convert_empty_point

test result: FAILED. 60 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.11s

Good catch! I overlooked this test.

[michaelkirk]

bors r+

[bors[bot]]

Build succeeded:

• wkt