Error calling `count()` on a null dtype struct field

[synchromic]

Example:

import daft
from daft import col
df = daft.from_pydict({"a": [
    {"c": None},
    None,
]})
df.show() # works
df.count(col("a").struct.get("c")).show() # breaks

Expected: a dataframe like {"c": [0]}

Actual:

thread '<unnamed>' panicked at src/arrow2/src/array/null.rs:91:9:
cannot set validity of a null array
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/conor/Documents/Programming/daft/Daft/daft/api_annotations.py", line 26, in _wrap
    return timed_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/analytics.py", line 198, in tracked_method
    return method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/dataframe/dataframe.py", line 2382, in show
    dataframe_display = self._construct_show_display(n)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/dataframe/dataframe.py", line 2339, in _construct_show_display
    for table in get_context().runner().run_iter_tables(builder, results_buffer_size=1):
  File "/Users/conor/Documents/Programming/daft/Daft/daft/runners/pyrunner.py", line 216, in run_iter_tables
    for result in self.run_iter(builder, results_buffer_size=results_buffer_size):
  File "/Users/conor/Documents/Programming/daft/Daft/daft/runners/pyrunner.py", line 211, in run_iter
    yield from results_gen
  File "/Users/conor/Documents/Programming/daft/Daft/daft/runners/pyrunner.py", line 318, in _physical_plan_to_partitions
    materialized_results = done_future.result()
                           ^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/runners/pyrunner.py", line 379, in build_partitions
    partitions = instruction.run(partitions)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/execution/execution_step.py", line 683, in run
    return self._aggregate(inputs)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/execution/execution_step.py", line 687, in _aggregate
    return [input.agg(self.to_agg, self.group_by)]
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/conor/Documents/Programming/daft/Daft/daft/table/micropartition.py", line 230, in agg
    return MicroPartition._from_pymicropartition(self._micropartition.agg(to_agg_pyexprs, group_by_pyexprs))
                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: cannot set validity of a null array

[Abyss-lord]

Hi @rchowell, I'm interested in working on this. Would you mind assigning it to me? BTW, could you please review following two PR when you have time?

1. fix(dataframe): preserve field order in from_pylist #6369
2. feat(expressions): support count(mode='all') without expr #6358