NPE when `go` closes over a local variable bound to nil

This throws a NPE after

Analysis: Compiler.java exhibits a quircky behaviour where for a local bound to nil, it'll return `true` for `lb.hasJavaClass()` but `lb.getJavaClass()` will return `nil`, which causes a NPE when we invoke `lb.getJavaClass().getName()`

Approach:
Don't assume `.getJavaClass` will return non-nil when `hasJavaClass` returns true

Patch: 0001-ASYNC-186-Apparently-Compiler.java-can-return-true-f.patch