Resolve runtime warning: invalid value encountered in cast

[ESadek-MO]

📰 Custom Issue

Written up from #5465.

lib/iris/tests/unit/analysis/test_MAX_RUN.py: 38 warnings
  /home/h05/achamber/.conda/envs/iris-dev/lib/python3.11/site-packages/numpy/core/numeric.py:407: RuntimeWarning: invalid value encountered in cast
    multiarray.copyto(res, fill_value, casting='unsafe')

lib/iris/tests/unit/analysis/regrid/test_RectilinearRegridder.py::Test__regrid__extrapolation_modes::test_method_result_types
  /net/home/h05/achamber/git/iris/lib/iris/analysis/_regrid.py:836: RuntimeWarning: invalid value encountered in cast
    data[tuple(index)] = interpolate(src_subset)

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