type hints: (trivial) add missing "Optional" qualifiers

[SomberNight]

In many cases with our type hinting, we do not explicitly specify that variables are allowed to be None when they have a default value of None.
This PR now adds the missing explicit "Optional" markers.

---

Consider:

class Foo(NamedTuple):
    field: str = None

This type-hinting is technically incorrect, we should type hint it as either one of these options:

• field: Optional[str] = None
• field: str | None = None

Thomas said he prefers the second, so str | None.
However I am not going to touch lines with existing Optionals, I am only fixing lines that are type-hinted incorrectly.

PyCharm has had soft-warnings about these lines for quite a while,
however when doing type-inference, it used to internally "fix" the type by allowing None.
Recently however it seems it is not doing the internal inference-fixing anymore, but assumes that path has type str (as we specified) and then make "incorrect" inferences:

class Foo(NamedTuple):
    field: str = None

def func(foo: Foo):
    assert foo.field is None
    print("function body")  # <<< PyCharm now says this code is unreachable

I have whole function bodies grayed out and marked as dead code due to this.

Example:

electrum/electrum/lnpeer.py

Line 2320 in c82a0d1

assert htlc_set.parent_set_key is None, f"Must not settle child {htlc_set=}"

, where htlc_set.parent_set_key is type-hinted as str, so the assert "must" always fail