Resolve runtime warning: divide by zero encountered in divide

[ESadek-MO]

📰 Custom Issue

Written up from #5465.

lib/iris/tests/unit/analysis/maths/test_divide.py::TestMasking::test_unmasked_div_zero
  /net/home/h05/achamber/git/iris/lib/iris/tests/unit/analysis/maths/test_divide.py:54: RuntimeWarning: divide by zero encountered in divide
    com = self.data_op(dat_b, dat_a)

lib/iris/tests/unit/analysis/maths/test_divide.py::TestMasking::test_unmasked_div_zero
  /net/home/h05/achamber/git/iris/lib/iris/analysis/maths.py:918: RuntimeWarning: divide by zero encountered in divide
    data = operation_function(lhs, rhs)

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