combine_first: conditional type-cast to rhs's dtype #10931
Description
test_combine_first is flaky. This is because the test randomly generates the test data, and if a chunk ends up being full of NaNs it will improperly trigger a FutureWarning in pandas that should only trigger when the whole Series is full of NaNs.
Reproducer linked below.
Additionally, if lhs is of dtype='datetime64[s]' and full of NaTs, while rhs has dtype='datetime64[ns]', the output dtype is rhs's dtype in pandas, but lhs's dtype in dask. This can't be fixed due to the dtype being eager in dask.