Don't change style when writing an empty string

[Nemo157]

fixes #30

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.52% :tada:

Comparison is base (efeaab0) 30.47% compared to head (df95202) 31.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ========================================== + Coverage 30.47% 31.00% +0.52% ========================================== Files 27 27 Lines 525 529 +4 ========================================== + Hits 160 164 +4 Misses 365 365 ``` | [Files Changed](https://app.codecov.io/gh/Nullus157/stylish-rs/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157) | Coverage Δ | | |---|---|---| | [ansi/src/ansi.rs](https://app.codecov.io/gh/Nullus157/stylish-rs/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-YW5zaS9zcmMvYW5zaS5ycw==) | `100.00% <100.00%> (ø)` | | | [html/src/html.rs](https://app.codecov.io/gh/Nullus157/stylish-rs/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-aHRtbC9zcmMvaHRtbC5ycw==) | `100.00% <100.00%> (ø)` | |

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