Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: egolearner

python/pyarrow/_dataset.pyx

@@ -2253,7 +2253,7 @@ cdef class CsvFileFormat(FileFormat):
         """
         cdef CsvFileWriteOptions opts = \
             <CsvFileWriteOptions> FileFormat.make_write_options(self)
-        if 'delimiter' not in kwargs:
+        if kwargs.get('delimiter') is None:
             kwargs['delimiter'] = opts.write_options.delimiter
         opts.write_options = WriteOptions(**kwargs)
         return opts