[pyserial] Fix read_all return type to not include None (#15797)

Author: bibekmhj

stubs/pyserial/serial/serialutil.pyi

@@ -164,6 +164,6 @@ class SerialBase(io.RawIOBase):
     def apply_settings(self, d: dict[str, Any]) -> None: ...
     def readinto(self, buffer: WriteableBuffer, /) -> int: ...  # returns int unlike `io.RawIOBase`
     def send_break(self, duration: float = 0.25) -> None: ...
-    def read_all(self) -> bytes | None: ...
+    def read_all(self) -> bytes: ...
     def read_until(self, expected: bytes = b"\n", size: int | None = None) -> bytes: ...
     def iread_until(self, expected: bytes = ..., size: int | None = ...) -> Generator[bytes]: ...