Fix whitespace lint error

[gliptak]

https://github.com/giampaolo/pyftpdlib/actions/runs/6646201388/job/18059152931

make[1]: Entering directory '/home/runner/work/pyftpdlib/pyftpdlib'
pyftpdlib/handlers.py:3607:1: W293 [*] Blank line contains whitespace
Found 1 error.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6d50bf7) 69.83% compared to head (7577133) 66.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #615 +/- ## ========================================== - Coverage 69.83% 66.66% -3.17% ========================================== Files 9 11 +2 Lines 3613 4164 +551 ========================================== + Hits 2523 2776 +253 - Misses 1090 1388 +298 ``` | [Files](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/615?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola) | Coverage Δ | | |---|---|---| | [pyftpdlib/handlers.py](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/615?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola#diff-cHlmdHBkbGliL2hhbmRsZXJzLnB5) | `70.28% <ø> (-0.21%)` | :arrow_down: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/615/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[gliptak]

@giampaolo please review