Related to #127701
When landing a seemingly unrelated CL, clang-tidy started failing with:
```
❌ Failures for clang-tidy on /b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:
/b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:57:10: error: unchecked access to optional value [bugprone-unchecked-optional-access,-warnings-as-errors]
return images_[image_index].frame_info.value();
^
/b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:154:13: error: unchecked access to optional value [bugprone-unchecked-optional-access,-warnings-as-errors]
switch (frame.frame_info->blend_mode) {
^
/b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:526:7: error: unchecked access to optional value [bugprone-unchecked-optional-access,-warnings-as-errors]
if (images_.back().frame_info->disposal_method ==
^
/b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:535:7: error: unchecked access to optional value [bugprone-unchecked-optional-access,-warnings-as-errors]
images_.back().frame_info->disposal_method ==
^
/b/s/w/ir/cache/builder/src/flutter/lib/ui/painting/image_generator_apng.cc:538:5: error: unchecked access to optional value [bugprone-unchecked-optional-access,-warnings-as-errors]
image->frame_info->required_frame = images_.size() - 1;
^
Suppressed 1772 warnings (1772 in non-user code).
```
This addresses those checks by making sure frame_info has a value.
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