print doesn't handle statements with empty bodies correctly

[Conduitry]

Downstream issue: sveltejs/svelte#6884

Generating an AST from something like while (foo()); bar(); and then serializing it back into a string results in while (foo()) bar();, presumably because node.body on the WhileStatement is undefined, and so nothing gets emitted for it, which is incorrect in this case. There are other statement types that are also affected by this (ForStatement, IfStatement, maybe others) and we should take a sweep through all of them to see which might be suffering from the same bug.