[fix] "TypeError: this.widget.markers is not iterable"

[slovnicki]

Added const <Marker>{} default value to GoogleMap.markers

Description

I was getting the following error:

The following JSNoSuchMethodError was thrown during a scheduler callback:
TypeError: this.widget.markers is not iterable

When the exception was thrown, this was the stack:
packages/flutter_google_maps/src/web/google_map.state.dart 638:33  <fn>
packages/flutter/src/scheduler/binding.dart 1144:15                [_invokeFrameCallback]
packages/flutter/src/scheduler/binding.dart 1090:9                 handleDrawFrame
packages/flutter/src/scheduler/binding.dart 998:5                  [_handleDrawFrame]
lib/_engine/engine/platform_dispatcher.dart 896:13                 invoke
lib/_engine/engine/platform_dispatcher.dart 145:5                  invokeOnDrawFrame
lib/_engine/engine.dart 257:45                                     <fn>

so this PR fixes it.

Related Issues

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Tests

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Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• [x] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] All existing and new tests are passing.
• [x] The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• [x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them?

• [x] No, no existing tests failed, so this is not a breaking change.