ModHookPreprocessor: discern between no return type and -> void

[Qubus0]

0 is the default for functions that don't specify type so we can't discern if a return type is set to void or if there is none at all.
currently we're dropping the type in those cases.

static func get_return_type_string(return_data: Dictionary) -> String:
    if return_data.type == 0:
        return ""

from get_script_method_list(), the .return is the same

func _ready():
{ "name": "", "class_name": &"", "type": 0, "hint": 0, "hint_string": "", "usage": 6 }

func _ready() -> void:
{ "name": "", "class_name": &"", "type": 0, "hint": 0, "hint_string": "", "usage": 6 }

the only way to know is to check if the method string contains -> void