Use an empty DataFrame for DataFrame indexing things & import/export cleanup

[codecov-commenter]

Codecov Report

Base: 95.83% // Head: 95.85% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (ec8d476) compared to base (5fc7ec1). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #21 +/- ## ========================================== + Coverage 95.83% 95.85% +0.01% ========================================== Files 10 10 Lines 817 820 +3 ========================================== + Hits 783 786 +3 Misses 34 34 ``` | [Impacted Files](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel) | Coverage Δ | | |---|---|---| | [src/operations/dataframes\_interface\_utils.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvZGF0YWZyYW1lc19pbnRlcmZhY2VfdXRpbHMuamw=) | `94.33% <ø> (ø)` | | | [src/operations/groupby.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvZ3JvdXBieS5qbA==) | `100.00% <ø> (ø)` | | | [src/operations/join.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvam9pbi5qbA==) | `100.00% <ø> (ø)` | | | [src/operations/join\_interface.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvam9pbl9pbnRlcmZhY2Uuamw=) | `100.00% <ø> (ø)` | | | [src/operations/operations.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvb3BlcmF0aW9ucy5qbA==) | `98.46% <ø> (ø)` | | | [src/operations/dataframes\_interface.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL29wZXJhdGlvbnMvZGF0YWZyYW1lc19pbnRlcmZhY2Uuamw=) | `89.58% <100.00%> (ø)` | | | [src/table/dtable.jl](https://codecov.io/gh/JuliaParallel/DTables.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel#diff-c3JjL3RhYmxlL2R0YWJsZS5qbA==) | `92.06% <100.00%> (+0.26%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaParallel)

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