[audiofileplayer] Address dart:unnecessary_non_null_assertion warning

[gilescm]

This PR addresses https://github.com/google/flutter.plugins/issues/134 by fixing dart:unnecessary_non_null_assertion warning so that the package is compatible with Flutter version 3.0.0. Warning details: The '!' will have no effect because the receiver can't be null. Try removing the '!' operator.

Warning about bindings in Flutter 3.0.0 release notes

[google-cla[bot]]

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[gilescm]

Hi @monkeyswarm would you be able to take a look at this PR?

[gilescm]

Hi @mit-mit would you be able to take a look at this PR?

[gilescm]

Hi @monkeyswarm just wondering if you've seen this?

[mr-mmmmore]

Hey @gilescm, the first answer states that you have to sign the Contributor License Agreement in order for your PR to be taken into account.

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[mr-mmmmore]

May be you should also add "[audiofileplayer]" to the issue's title.

[gilescm]

Hi @mr-mmmmore , good idea I’ve updated the title thanks! Regarding the CLA I completed it shortly after getting that message, you can see it’s completed in the Checks tab

[monkeyswarm]

Added comment about backwards compatibility; not sure what the best practice is.