selftests/bpf: Add test cases with CONST_PTR_TO_MAP null checks

A test requires the following to happen:
  * CONST_PTR_TO_MAP value is checked for null
  * the code in the null branch fails verification

Add test cases:
* direct global map_ptr comparison to null
* lookup inner map, then two checks (the first transforms
  map_value_or_null into map_ptr)
* lookup inner map, spill-fill it, then check for null
* use an array of ringbufs to recreate a common coding pattern [1]

[1] https://lore.kernel.org/bpf/CAEf4BzZNU0gX_sQ8k8JaLe1e+Veth3Rk=4x7MDhv=hQxvO8EDw@mail.gmail.com/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <isolodrai@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250609183024.359974-4-isolodrai@meta.com

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