bump @pybricks/firmware to 5.0.0

This contains a breaking change where main.py is now optional so we
have to handle that case.

src/firmware/sagas.ts

@@ -201,11 +201,17 @@ function* loadFirmware(
     const firmwareBase = yield* call(() => reader.readFirmwareBase());
     const metadata = yield* call(() => reader.readMetadata());
 
-    // if a user program was not given, then use main.py from the frimware.zip
+    // if a user program was not given, then use main.py from the firmware.zip
     if (program === undefined) {
         program = yield* call(() => reader.readMainPy());
     }
 
+    // REVISIT: the firmware may eventually be changed to allow no main.py
+    // for now, ensure there is a program even if it does nothing
+    if (!program) {
+        program = '';
+    }
+
     if (![5, 6].includes(metadata['mpy-abi-version'])) {
         yield* put(
             didFailToFinish(