Resolve runtime warning: invalid value encountered in sqrt.

[ESadek-MO]

📰 Custom Issue

Written up from #5465.

lib/iris/tests/test_plot.py::Test2dPoints::test_circular_changes
  /home/h05/achamber/.conda/envs/iris-dev/lib/python3.11/site-packages/matplotlib/collections.py:963: RuntimeWarning: invalid value encountered in sqrt
    scale = np.sqrt(self._sizes) * dpi / 72.0 * self._factor

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