Fixes an Android native unit test and Android example issue

[matux]

Description of the change

An Android unit test started to fail with Flutter SDK 3.7.

The function encodeErrorEnvelopeWithStacktrace on our mocked MethodCodec is being called without a stack trace, this call comes from the Flutter system. I've tracked this down to:

• https://api.flutter.dev/javadoc/io/flutter/plugin/common/MethodCodec.html#encodeErrorEnvelopeWithStacktrace(java.lang.String,java.lang.String,java.lang.Object,java.lang.String)

Where "errorStacktrace - Platform stacktrace for the error. possibly null."

So it seems it is correct that this may be null. I've added a check.

The library and example works fine regardless of the Flutter SDK version.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Maintenance
• [ ] New release

Related issues

• This issue isn't being tracked.

Checklists

Development

• [x] Lint rules pass locally
• [x] The code changed/added as part of this pull request has been covered with tests
• [x] All tests related to the changed code pass in development

Code review

• [x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [x] "Ready for review" label attached to the PR and reviewers assigned
• [x] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer