Fix casting of list of textruns

[stefannikolei]

Directly casting is not possible. That's why using the Cast method is the easiest way

Fixes #332

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80%. Comparing base (ac272a2) to head (ce0f50c). Report is 13 commits behind head on main.

Files	Patch %	Lines
...c/ImageSharp.Drawing/Processing/RichTextOptions.cs	0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #333 +/- ## =================================== Coverage 80% 80% =================================== Files 97 97 Lines 4960 4971 +11 Branches 886 888 +2 =================================== + Hits 4011 4023 +12 + Misses 754 753 -1 Partials 195 195 ``` | [Flag](https://app.codecov.io/gh/SixLabors/ImageSharp.Drawing/pull/333/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/SixLabors/ImageSharp.Drawing/pull/333/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | `80% <0%> (+<1%)` | :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=SixLabors#carryforward-flags-in-the-pull-request-comment) to find out more.

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[JimBobSquarePants]

Would direct casting not work if we explicitly set an empty array as the default value?

[JimBobSquarePants]

Thanks @stefannikolei for this. I'm going to close it though as I've added a fix to #335 that assigns the empty array.