Goal of this PR

Fix handling of empty and nil slices. There are scenarios where empty slices get turned into slices of 1 element, and where nil slices cause a panic.

This PR fixes these scenarios.

How did I test it?

Added unit tests that demonstrate various scenarios, some of which fail before the fix and pass with the fix.
Notably:

• TestDecoder_UnionInterfaceArrayEmpty
  • empty array of ints in union decoded as a slice of int with 1 element instead of empty slice
• TestDecoder_UnionArrayRecordEmpty
  • empty array of records in union decoded to non-nil pointer to nil slice as nil slice instead of empty slice
• TestDecoder_UnionInterfaceArrayRecordEmpty
  • empty array of records in union decoded as a slice of records with 1 element instead of empty slice
• TestEncoder_UnionInterfaceUnregisteredArrayNull
  • encoding a nil array in map[string]any panicked instead of encoding an empty slice