:snake::art::fire::bug: Fix UnboundLocalError bug

[evanroyrees]

Remove unnecessary nesting leading to UnboundLocalError bug.

Fixes #324

UnboundLocalError resulted from trying to update binning_checkpoints dataframe when it was actually not available (occurs when --cache is not provided). Now variable is initialized accordingly to remove this error.

PR checklist

• [x] This comment contains a description of changes (with reason).
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [x] Have you followed the pipeline conventions in the contribution docs

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.29% :tada:

Comparison is base (04b2926) 27.31% compared to head (521ac53) 27.60%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #325 +/- ## ========================================== + Coverage 27.31% 27.60% +0.29% ========================================== Files 50 50 Lines 5730 5730 ========================================== + Hits 1565 1582 +17 + Misses 4165 4148 -17 ``` | [Flag](https://app.codecov.io/gh/KwanLab/Autometa/pull/325/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/KwanLab/Autometa/pull/325/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab) | `27.60% <0.00%> (+0.29%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/KwanLab/Autometa/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab) | Coverage Δ | | |---|---|---| | [autometa/binning/large\_data\_mode.py](https://app.codecov.io/gh/KwanLab/Autometa/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab#diff-YXV0b21ldGEvYmlubmluZy9sYXJnZV9kYXRhX21vZGUucHk=) | `0.00% <0.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/KwanLab/Autometa/pull/325/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=KwanLab)

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