Fix panic when receiving empty strings in rpc calls

Receiving an empty string in an RPC call currently panics.

When `length` is zero, a call to the `alloc` function (as implemented in `artiq/firmware/runtime/session.rs`) returns a null pointer. Constructing a `CMutSlice` from a null pointer panics.
A `CMutSlice` consists of a pointer and the length. Rust's documentation of the `core::ptr` module states: "The canonical way to obtain a pointer that is valid for zero-sized accesses is `NonNull::dangling`."
This commits adds a check for the length of a string received in an RPC call. Only for lengths greater than zero a memory allocation is performed. For zero-length strings, a dangling pointer is used.

Test plan:
Invoke the following experiment, which returns an empty string over RPC:
```
class ReturnEmptyString(artiq.experiment.EnvExperiment):
    def build(self):
        self.core: Core = self.get_device("core")

    @kernel
    def run(self):
        x = self.do_rpc()
        print(x)

    @rpc
    def do_rpc(self) -> TStr:
        return ""
```

Signed-off-by: Sven Over (Oxford Ionics) <sven.over@oxionics.com>

artiq/firmware/libproto_artiq/rpc_proto.rs

@@ -36,8 +36,12 @@ unsafe fn recv_value<R, E>(reader: &mut R, tag: Tag, data: &mut *mut (),
         Tag::String | Tag::Bytes | Tag::ByteArray => {
             consume_value!(CMutSlice<u8>, |ptr| {
                 let length = reader.read_u32()? as usize;
-                *ptr = CMutSlice::new(alloc(length)? as *mut u8, length);
-                reader.read_exact((*ptr).as_mut())?;
+                if length > 0 {
+                    *ptr = CMutSlice::new(alloc(length)? as *mut u8, length);
+                    reader.read_exact((*ptr).as_mut())?;
+                } else {
+                    *ptr = CMutSlice::new(core::ptr::NonNull::<u8>::dangling().as_ptr(), 0);
+                }
                 Ok(())
             })
         }