Resource empty text: content-declared, and null counts as empty

A `.[] | {...}` jq over an empty list yields no output, which arrived
as null and rendered nothing - redoal's Releases feature was a bare
heading. Null now reads as the same silence as [], and ResourceSpec
gains `empty:` so content can word it ("Nothing released yet");
default stays "Nothing here yet."

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/content.rs

@@ -202,6 +202,12 @@ pub struct ResourceSpec {
     /// `.[] | {name, url: .html_url}`. `None` returns it as-is.
     #[serde(default)]
     pub jq: Option<String>,
+    /// What to say when the resource yields nothing - an empty list,
+    /// or `null` (a `.[] | ...` jq over an empty list produces no
+    /// output at all). Defaults to "Nothing here yet."; content sets
+    /// it where the silence needs a voice ("Nothing released yet").
+    #[serde(default)]
+    pub empty: Option<String>,
 }
 
 /// Where a resource's live data comes from. `Kv` (a NATS KV bucket