[in_app_purchase] Fix AppStore cancel payment modal

[dennis-tra]

Description

When the user cancels the payment dialog there is no transactionIdentifier this means the transaction cannot be finished in the current implementation and therefore not initiated again.

I haven't polished the PR to be included here, but just wanted to give a heads up. Feel free to close it if it does not meet your standards.

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[LHLL]

I think it makes sense. Can you polish it up and add some tests?

[dennis-tra]

Hi @LHLL, unfortunately I don't have much time to spare currently, so can't promise any ETA, but it's on my list.

[craigomac]

This is an incredibly annoying and basic flaw with the plugin. Thanks for your PR!

[dennis-tra]

Just created a new PR which updates the tests: #3106