`INSTRUMENTED_FUNCTION_CALL_EX` should set `arg0` to `MISSING` instead of `None` if argument is absent

[gaogaotiantian]

Bug report

Bug description:

I should've caught this in #116626 but somehow I missed it. INSTRUMENTED_FUNCTION_CALL_EX sets arg0 to None now, instead of MISSING, when there's no argument.

def f(a=1, b=2):
    return a + b
empty_args = []
f(*empty_args)  # arg0 will be None

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-116735: Use MISSING for CALL event if argument is absent #116737
• [3.12] gh-116735: Use MISSING for CALL event if argument is absen… #116873