fix: handle non-numeric values in _is_nan_value to prevent save failure (closes #219)

[botbikamordehai2-sketch]

What

When saving labels in the napari plugin, the _is_nan_value function in misc.py attempts to call np.isnan() on values that may be non-numeric (e.g., strings, tuples in MultiIndex columns). This raises an unhandled exception that prevents the save dialog from completing, leaving no feedback to the user. The traceback shows the error originates from the write path.

Fix

Modify _is_nan_value to check if the input is numeric before calling np.isnan(). Non-numeric values are returned as False (they are not NaN). This ensures the helper function _array_has_nan works correctly on mixed-type arrays such as those with MultiIndex column labels.

Closes #219

[C-Achard]

As confirmed in #219 by the user after investigation, the error was due to a duplicate bodypart in config.yaml being improperly handled.

The altered function in this PR is not related in any way to saving or writing and is used only in color display checks.
The issue is properly fixed in #220.