bug: ST_Azimuth should return null instead of 0.0 for identical points

[petern48]

Came up in this conversation: https://github.com/apache/sedona-db/pull/183#discussion_r2404840176

tldr; Sedona return 0.0 whereas it's supposed to return null like in PostGIS. an output of 0.0 is more for indicating "north"