Added test to check for nil semantics of uninitialized local variables and empty blocks.

[smarr]

Local variables can be uninitialized on first access.
At this point, they should have nil as their value.

This is standard Smalltalk semantics, allowing uninitialized access, but ensuring safety by having a well known initial value.

[smarr]

The latest commit also adds a test that empty blocks return nil (#36).

[smarr]

Status:

• SOM (java) Fix semantics of empty blocks som-java#19
• TruffleSOM Added tests for uninitialized locals and empty blocks TruffleSOM#40
• SOM++ Fix semantics of empty blocks and implement basic interpreter tests SOMpp#8
• CSOM Fix semantics of empty blocks CSOM#18
• RTruffleSOM Updated core-lib RTruffleSOM#4