Catch missing value

[cthoyt]

Since pandas implements missing values as floats, this can give exceptions like

Traceback (most recent call last):
  File "/Users/cthoyt/.virtualenvs/biopragmatics/bin/biomappings", line 10, in <module>
    sys.exit(main())
             ~~~~^^
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/core.py", line 1462, in __call__
    return self.main(*args, **kwargs)
           ~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/core.py", line 1383, in main
    rv = self.invoke(ctx)
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/core.py", line 1850, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/core.py", line 1246, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/core.py", line 814, in invoke
    return callback(*args, **kwargs)
  File "/Users/cthoyt/.virtualenvs/biopragmatics/lib/python3.14/site-packages/click/decorators.py", line 46, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/Users/cthoyt/dev/sssom-curator/src/sssom_curator/repository.py", line 458, in main
    merge(obj, directory=sssom_directory)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cthoyt/dev/sssom-curator/src/sssom_curator/export/merge.py", line 90, in merge
    write_owl(msdf, file)
    ~~~~~~~~~^^^^^^^^^^^^
  File "/Users/cthoyt/dev/sssom-py/src/sssom/writers.py", line 235, in write_owl
    graph = to_owl_graph(msdf)
  File "/Users/cthoyt/dev/sssom-py/src/sssom/writers.py", line 280, in to_owl_graph
    graph = to_rdf_graph(msdf=msdf)
  File "/Users/cthoyt/dev/sssom-py/src/sssom/writers.py", line 374, in to_rdf_graph
    return MappingSetRDFConverter(hydrate=hydrate, ccp=msdf.converter).msdf_to_rdf(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        msdf, hydrate=hydrate
        ^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/cthoyt/dev/sssom-py/src/sssom/rdf_internal.py", line 879, in msdf_to_rdf
    self._mapping_to_rdf(graph, ms_node, row, hydrate)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cthoyt/dev/sssom-py/src/sssom/rdf_internal.py", line 915, in _mapping_to_rdf
    mapping_node = self.mapping_converter.dict_to_rdf(graph, mapping)
  File "/Users/cthoyt/dev/sssom-py/src/sssom/rdf_internal.py", line 583, in dict_to_rdf
    for value in self._get_multi_values(v):
                 ~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/Users/cthoyt/dev/sssom-py/src/sssom/rdf_internal.py", line 974, in _get_multi_values
    return cast(List[Any], value.split("|"))
                           ^^^^^^^^^^^
AttributeError: 'float' object has no attribute 'split'

i have to put more time to understand how all these calls are made so I can write a unit test