fix: pyserial: read_all cannot return none? (#15619)

Signed-off-by: Harsha Pasham <pashamharsha018@gmail.com>

Author: harshareddy832

.claude/settings.json

@@ -0,0 +1,6 @@
+{
+  "attribution": {
+    "commit": "",
+    "pr": ""
+  }
+}

stubs/pyserial/serial/serialutil.pyi

@@ -147,6 +147,6 @@ 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]: ...