fix: Make string literals throw again, and test optional enums

[mrousavy]

When using a single string literal in Nitrogen, it must throw again because it is simply ambiguous between a string and an enum. We don't know what it is, so we need to tell the user to be more explicit.

Also fixes:

• Optionals in Swift can be passed through as is if possible - no optional lambda try/unwrapping
• Disable codegen for Android (it failed because I don't use it)