Prevent zero-length metadata to be created

This isn't the most idiomatic way, but I'll change it in another moment.

src/main.rs

@@ -235,6 +235,17 @@ impl eframe::App for App {
 
                         ui.add_space(10.0);
                         if ui.button("add note").clicked() {
+                            // FIXME(sofia@git.sofiaritz.com): Use a more idiomatic way
+                            let metadata = if let Some(metadata) = self.metadata_buffer.clone() {
+                                if metadata.is_empty() {
+                                    None
+                                } else {
+                                    Some(metadata)
+                                }
+                            } else {
+                                None
+                            };
+
                             let note = Note::Decrypted {
                                 id: SystemTime::now()
                                     .duration_since(SystemTime::UNIX_EPOCH)
@@ -244,7 +255,7 @@ impl eframe::App for App {
                                 title: self.title_buffer.clone(),
                                 metadata: NoteMetadata {
                                     date: SystemTime::now(),
-                                    arbitrary: self.metadata_buffer.clone(),
+                                    arbitrary: metadata,
                                 },
                                 text: self.text_buffer.clone(),
                             };