Default values in uninitialized arrays can contain null

[bamarsha]

Describe the bug

The default values for a type T given by (new T[1])[0] or the equivalent library function Default<T>() do not match the documented values here:

https://docs.microsoft.com/en-us/quantum/user-guide/language/expressions#array-creation

Expected behavior

The default values should match the documented values.

Actual behavior

• Default<String>() is null but should be ""
• Default<Result>() is null but should be Zero
• Default<Range>() is 0..1..0 but should be 1..1..0
• Default<'T[]>() is null but should be new 'T[0]
• Default<Unit>() is null but should be ()

Additional context
See microsoft/qsharp-compiler#589 for more examples.

[bamarsha]

I started working on this on branch samarsha/default-values. The handling of default values for Q# types has several problems. One is that the logic for creating default values is duplicated in both QArray and generated UDTs. It also assumes that C#'s default maps to Q# defaults, which in many cases it does not. I think we will need to unify the way QArray and UDTs create default values, so there's one place that we can define the correct Q# defaults.

[bamarsha]

Another option is to just remove the default constructor on UDTs so all the default value logic is in QArray... I'm not sure which places the default constructor is used from.