Bug fix in pairwise_linestring_intersection

[isVoid]

Description

This PR fixes a bug in pairwise_linestring_intersection, where point_flags is incorrectly used to remove points in points array due to leftover results from find_duplicate_points.

Previously, the code incorrectly assumed that point_flags will always be written to prior to merge_point_on_segment kernel, however, this may not be true if the result segment array is empty, and the kernel launch could be evaded, leading to that the second remove_if operates on the "leftover" from the previous point_flag ramnant.

This fix is to only run segment cleanup kernels if there is at least one segment result in the output. This not only fixes #1067 , but also serve as an optimization to avoid unnecessary kernel launching.

It should be pointed out that the result from python isn't exactly 1-1 matching with geopandas, as geopandas returns

0    MULTIPOINT (1.00000 1.00000, 0.00000 0.00000)
1    MULTIPOINT (1.00000 1.00000, 0.00000 0.00000)
2    MULTIPOINT (1.00000 1.00000, 0.00000 0.00000)

While cuspatial returns the result in 6 point rows with geometry offsets.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[harrism]

/merge