GH-54570: [C++][Compute] Fix segfault casting null list with empty child to fixed_size_list

[ShivanshhhG]

Rationale for this change

When casting a ListArray, LargeListArray, or ListViewArray to FixedSizeListArray, if the input array contains only null lists with an empty child array (which frequently occurs in IPC streams or deserialized batches from tools like Polars to_arrow()), cast_values->length is 0.

Previously, BuildTakeIndices populated index 0 for every null slot (builder.UnsafeAppend(0)). When Take was called with TakeOptions::NoBoundsCheck(), it attempted to access index 0 on an empty child array whose buffers may be null or unallocated, dereferencing a null pointer in FixedWidthTakeImpl and causing a segmentation fault (GH-54570).

What changes are included in this PR?

1. If cast_values->length == 0, bypass Take entirely and directly create the output child array using MakeArrayOfNull(child_type, in_array.length * list_size, ctx->memory_pool()).
2. In BuildTakeIndices, emit null indices (builder.UnsafeAppendNull()) for null list slots instead of index 0.
3. Add regression tests in cpp/src/arrow/compute/kernels/scalar_cast_test.cc verifying that casting null lists with empty children (including unallocated child buffers) to FixedSizeList succeeds and produces the expected null list array.

Are these changes tested?

Yes, unit tests added to cpp/src/arrow/compute/kernels/scalar_cast_test.cc under TEST(Cast, ListToFSL).

Are there any user-facing changes?

No (fixes crash/segfault on valid spec-compliant input).

• GitHub Issue: [C++][Compute] Casting an all-null list array read from IPC to fixed_size_list segfaults (Take with out-of-bounds index on an empty child) #54570

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