fixed bug when comparing string fill values

This PR fixes a bug introduced by #1152 that prevents Iris being able to load certain files it used to have not issues with. The problem arises with netcdf files with variables containing strings. If the fill values are strings calling `np.isnan(fill_value)` raises a TypeError. #1152 added an `np.isnan()` because `np.nan`s do not compare equal. This PR covers both cases.