[GH-3359] Return 2D results from ST_NDims, ST_CoordDim and ST_Zmflag for empty geometries

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• Yes, and the PR name follows the format [GH-XXX] my subject. Closes ST_NDims, ST_CoordDim and ST_ZMFlag throw NullPointerException on empty geometries #3359

What changes were proposed in this PR?

Functions.nDims and Functions.zmFlag dereference Geometry.getCoordinate(), which JTS returns as null for empty geometries, so ST_NDims, ST_CoordDim and ST_Zmflag failed the whole query with a NullPointerException whenever the column contained an empty geometry.

• Guard the null coordinate in both functions and report empty geometries as 2D: ST_NDims / ST_CoordDim return 2 and ST_Zmflag returns 0. This matches PostGIS (ST_NDims('POINT EMPTY') = 2, ST_Zmflag('POINT EMPTY') = 0) and the existing ST_HasZ / ST_HasM handling, which already null-check the same call.
• Spark, Flink and Snowflake all delegate to the common implementation, so the fix covers every engine.
• Document the empty-geometry result on the ST_NDims, ST_CoordDim and ST_Zmflag pages and add a release-notes entry.

How was this patch tested?

• New unit tests in FunctionsTest: nDims, nDimsEmptyGeometries and testZmFlagEmptyGeometries cover every empty geometry type from WKT plus factory-created empties. Both empty-geometry tests fail with the reported NPE before the fix and pass after it; FunctionsTest runs 308 tests green.
• New Spark SQL tests in functionTestScala (ST_NDims and ST_CoordDim with empty geometries, ST_Zmflag with empty geometries); the full suite passes on Spark 3.5 / Scala 2.12 (239 tests).

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation.