accounts/abi/bind: add void if no return args specified (#21002)

* accounts/abi/bind: add void if no return args specified

Currently the java generator generates invalid input on pure/view functions
that have no return type. e.g. `function f(uint u) view public {}`
This is not a problem in practice as people rarely ever write functions like this.

* accounts/abi/bind: use elseif instead of nested if