Segfault when an optional value is passed to a Python-implemented function #113

What happens

Passing a CEL optional to a function whose overload has a Python impl
kills the process (SIGSEGV, exit 139) before the function is called. The same
happens when the argument is a map holding an optional. A Python try cannot
catch it.

Reproduce

import sys
from cel_expr_python import cel
from cel_expr_python.ext import ext_optional

def show(x):
    return "ok"

env = cel.NewEnv(
    variables={"m": cel.Type.DYN},
    extensions=[ext_optional.ExtOptional()],
    functions=[cel.FunctionDecl("show", [
        cel.Overload("show_any", cel.Type.STRING, [cel.Type("any")], impl=show),
    ])],
)
act = env.Activation({"m": {"a": 1}})
print(env.compile("show(m)").eval(act).value(), flush=True)      # ok
print(env.compile(sys.argv[1]).eval(act).value(), flush=True)

Expected

An evaluation error, as the list case already gives, or the optional handed to
the function as a value.

Likely cause

CelValueToPyObject has no case for an optional and returns null with a
Python error set. PyCelFunctionAdapter::Invoke puts that null into the
argument tuple and calls the function without checking it.

Versions

cel-expr-python 0.1.3 from PyPI, and main at 45eefa7 built from source.
CPython 3.14.6, macOS arm64.