Fix zero prio tasks when all async tasks have default (no) arguments

rtic-macros/ui/task-no-prio.rs

@@ -0,0 +1,19 @@
+#![no_main]
+
+#[rtic_macros::mock_app(device = mock)]
+mod app {
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn init(_: init::Context) -> (Shared, Local) {}
+
+    #[idle]
+    fn idle(_: idle::Context) -> ! {}
+
+    #[task]
+    async fn task1(_: task1::Context) {}
+}

rtic-macros/ui/task-no-prio.stderr

@@ -0,0 +1,5 @@
+error: Async task "task1" has priority 0, but `#[idle]` is defined. 0-priority async tasks are only allowed if there is no `#[idle]`.
+  --> ui/task-no-prio.rs:18:14
+   |
+18 |     async fn task1(_: task1::Context) {}
+   |              ^^^^^