Tst interval index NaN uniqueness
add test cases for interval-indexer with unique NaN value and non-unique NaN values.
Thanks @r-toroxel is this not currently tested or is it one of the open issues that are fixed on master

is this related to #41831 and the associated open PR #41863 @simonjayhawkins yes, these were not tested before and seem to be related to #41831. thanks @r-toroxel  @jbrockmendel does this close #41831 ? @jreback I'm afraid it does not fix the bug and hence close the issue. just a test added.