[BUG] [CPP-UE4] Fix codegen for nested container types

[madrazzl3]

Fixed code generation for nested container type (list of lists / map of lists) for UE4.

@wing328 @Kahncode

Testcase for testing:
https://gist.github.com/madrazzl3/b2d9ab436b16fa82151816d31f73468b

Fix #12746

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[Kahncode]

Hello @madrazzl3. Thanks for the fix, it looks quite good. Could you share the difference in the generated code?

[madrazzl3]

> #12746

Hi, there is one from issue ticket:

Before:

TOptional<TMap<FString, TArray>> PetsMapExample;

After:

TOptional<TMap<FString, TArray<Pet>>> PetsMapExample;

Similar for List type:

Before:

TOptional<TArray<TArray>> PetsArrayExample;

After:
TOptional<TArray<TArray<Pet>>> PetsArrayExample;

[Kahncode]

Great! Looks good to me.