BUG: Incorrect variable window bounds for first row when offset cover all rows
- [x] closes #42753 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

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Allow for cases in which the end bound of the variable window for the first row extends past the last row, that is, cases in which the offset is large enough to cover all the rows.