Fix failure when fill_opacity of a drawing was set to None rather than not set

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (bd9a0a0) 28.20% compared to head (bca9701) 28.20%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8 +/- ## ======================================= Coverage 28.20% 28.20% ======================================= Files 41 41 Lines 3240 3240 ======================================= Hits 914 914 Misses 2326 2326 ``` | [Impacted Files](https://app.codecov.io/gh/jennis0/burdoc/pull/8?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [...c/processors/pdf\_load\_processor/drawing\_handler.py](https://app.codecov.io/gh/jennis0/burdoc/pull/8?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2J1cmRvYy9wcm9jZXNzb3JzL3BkZl9sb2FkX3Byb2Nlc3Nvci9kcmF3aW5nX2hhbmRsZXIucHk=) | `12.26% <0.00%> (ø)` | | | [src/burdoc/utils/compare.py](https://app.codecov.io/gh/jennis0/burdoc/pull/8?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2J1cmRvYy91dGlscy9jb21wYXJlLnB5) | `0.00% <0.00%> (ø)` | |

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