Handle missing MAPAXES export when mapaxes is absent

[yasser508]

Make rd_grid_alloc_mapaxes_kw() handle grids without mapaxes.

I added a regression test for the case where rd_grid_alloc_mapaxes_kw() is called for a grid with no MAPAXES data, which was the issue in #1117.

Then I changed the implementation so it returns an empty keyword instead of a size-6 keyword with null backing data.

[Copilot]

Pull request overview

This PR fixes rd_grid_alloc_mapaxes_kw() to behave safely for grids that have no MAPAXES data (i.e., grid->mapaxes == NULL), addressing the segfault reported in #1117 by returning an empty keyword instead of a size-6 keyword with NULL backing storage.

Changes:

• Added a regression test asserting rd_grid_alloc_mapaxes_kw() returns an empty keyword when called on a grid without mapaxes.
• Updated rd_grid_alloc_mapaxes_kw() to return size 0 when mapaxes is absent (and size 6 when present).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/resdata/tests/rd_grid_export.cpp	Adds regression coverage for allocating MAPAXES kw on a grid with no mapaxes.
lib/resdata/rd_grid.cpp	Makes rd_grid_alloc_mapaxes_kw() return a 0-sized keyword when grid->mapaxes is NULL, preventing NULL-data access via size-driven loops.

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