Avoid generating empty thread_locals

[FranciscoTGouveia]

A new IsConvertableGlobalVarDecl function was introduced to determine whether the global variable declaration would emit any code.
This was needed because empty thread locals were previously being generated (i.e., thread_local!()).

[nunoplopes]

This patch seems weird. Isn't the code exactly the same before/after, just less efficient?

[FranciscoTGouveia]

Before, we would have:

thread_local!(pub static x_0: Value<i32> = <Value<i32>>::default());
thread_local!();

Whereas now, we just have:

thread_local!(pub static x_0: Value<i32> = <Value<i32>>::default());

This has no performance implications, but it avoids generating meaningless statements.