FIX-#4993: Return _default_to_pandas in df.attrs

[pyrito]

Signed-off-by: Karthik Velayutham vkarthik@ponder.io

What do these changes do?

We forgot to return the return value from _default_to_pandas 🤣

• commit message follows format outlined here
• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves BUG: dataframe.attrs is None by default #4993
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date
• added (Issue Number: PR title (PR Number)) and github username to release notes for next major release

[mvashishtha]

LGTM, thanks @pyrito !

[codecov]

Codecov Report

Merging #4995 (34c3f0e) into master (f727c04) will decrease coverage by 15.13%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #4995       +/-   ##
===========================================
- Coverage   84.87%   69.74%   -15.14%     
===========================================
  Files         267      268        +1     
  Lines       19746    20029      +283     
===========================================
- Hits        16760    13969     -2791     
- Misses       2986     6060     +3074     

Impacted Files	Coverage Δ
modin/pandas/dataframe.py	91.59% <100.00%> (+0.10%)	⬆️
...odin/experimental/core/storage_formats/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...din/experimental/core/execution/native/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...erimental/core/storage_formats/omnisci/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
.../core/execution/native/implementations/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...tive/implementations/omnisci_on_native/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...e/implementations/omnisci_on_native/io/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...mentations/omnisci_on_native/dataframe/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...tations/omnisci_on_native/partitioning/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...mentations/omnisci_on_native/calcite_serializer.py	0.00% <0.00%> (-98.71%)	⬇️
... and 84 more

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