GDScript: Error when assigning return value of void function

This also makes built-in method calls empty the return value when the
method is void, to avoid keeping returning a garbage value in such case.

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_free_call.gd

@@ -0,0 +1,4 @@
+func test():
+	var obj
+	obj = Node.new()
+	print(obj.free())

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_free_call.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: test()
+>> runtime/errors/use_return_value_of_free_call.gd
+>> 4
+>> Trying to get a return value of a method that returns "void"

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_void_builtin_method_call.gd

@@ -0,0 +1,4 @@
+func test():
+	var value
+	value = []
+	print(value.reverse())

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_void_builtin_method_call.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: test()
+>> runtime/errors/use_return_value_of_void_builtin_method_call.gd
+>> 4
+>> Trying to get a return value of a method that returns "void"

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_void_native_method_call.gd

@@ -0,0 +1,4 @@
+func test():
+	var obj
+	obj = RefCounted.new()
+	print(obj.notify_property_list_changed())

modules/gdscript/tests/scripts/runtime/errors/use_return_value_of_void_native_method_call.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: test()
+>> runtime/errors/use_return_value_of_void_native_method_call.gd
+>> 4
+>> Trying to get a return value of a method that returns "void"