Fix: Unsupported `AppleSystemUIFont` font

[joseph-grabinger]

When exporting a Text widget that uses a TextStyle provided through a TextTheme (on macOS & iOS) the following error is thrown:

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Exception: Unsupported Font: .AppleSystemUIFont
#0      TextStyleConverter.resolveFont (package:flutter_to_pdf/args/text_style.dart:66:11)
#1      TextStyleConverter.toPdfTextStyle (package:flutter_to_pdf/args/text_style.dart:39:21)
#2      TextConverter.toPdfWidget (package:flutter_to_pdf/widgets/text.dart:22:29)
#3      ExportInstance.matchWidget (package:flutter_to_pdf/export_instance.dart:176:40)
#4      ExportInstance._visit (package:flutter_to_pdf/export_instance.dart:58:29)

Closes #78.

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Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.46%. Comparing base (3950e24) to head (2011d03). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/args/text_style.dart	70.00%	3 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #92 +/- ## ========================================== - Coverage 85.52% 85.46% -0.07% ========================================== Files 58 58 Lines 898 901 +3 ========================================== + Hits 768 770 +2 - Misses 130 131 +1 ```

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[RenatoCesarF]

is this working on version 0.2.2? I cant import my font, and if i do not import, get this error

[joseph-grabinger]

@RenatoCesarF this will be present in version 0.2.3