Fix thread start with no user data when target has no default argument

[RandomShaper]

This fixes the case where no user data is specified to start a thread with a target method taking one parameter with no default value.

This script has been used the test the outcomes before and after this PR:

var thread: Thread

func _ready():
	thread = Thread.new()
	for i in range(1, 5):
		print("Case %d" % i)
		thread.start(self, "do_stuff_%d" % i)
		prints("Passing nothing", "WORKED" if thread.wait_to_finish() else "FAILED")
		thread.start(self, "do_stuff_%d" % i, "User data")
		prints("Passing string", "WORKED" if thread.wait_to_finish() else "FAILED")
		thread.start(self, "do_stuff_%d" % i, 666)
		prints("Passing integer", "WORKED" if thread.wait_to_finish() else "FAILED")

func do_stuff_1(optional_var: int):
	return true

func do_stuff_2(optional_var: int = 0):
	return true

func do_stuff_3(optional_var):
	return true

func do_stuff_4(optional_var = null):
	return true

These are the results:

• Case 1 (optional_var: int)

  • Before & after
    • Passing nothing FAILED (Too Few Arguments)
    • Passing string FAILED (Invalid Argument #0)
    • Passing integer WORKED

• Case 2 (optional_var: int = 0)

  • Before & after
    • Passing nothing WORKED
    • Passing string FAILED (Invalid Argument #0)
    • Passing integer WORKED

• Case 3 (optional_var)

  • Before
    • Passing nothing FAILED (Too Few Arguments)
    • Passing string WORKED
    • Passing integer WORKED
  • After
    • Passing nothing WORKED
    • Passing string WORKED
    • Passing integer WORKED

• Case 4 (optional_var = null)

  • Before & after
    • Passing nothing WORKED
    • Passing string WORKED
    • Passing integer WORKED

Fixes #50692.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.