Optimize ST_IsEmpty by avoiding deserialization

[Algunenano]

As <subject> says, there is no need to deserialize a geometry to know if it's empty.

EXPLAIN ANALYZE Select ST_IsEmpty(the_geom_webmercator) from benchmark_7773a711c8441d4b494a51fd9feebeac7a9b9c734619398620293;

Before:

number of transactions actually processed: 98
latency average = 204.691 ms

After:

number of transactions actually processed: 120
latency average = 167.220 ms

GH PR: ​https://github.com/postgis/postgis/pull/442

[algunenano]

In 17603:

ST_IsEmpty: Speed up by avoiding deserialization

Closes #4453
Closes ​https://github.com/postgis/postgis/pull/442