disallow empty attributes in entries

src/database/entry.rs

@@ -60,6 +60,9 @@ impl TryFrom<&Entry> for models::Entry {
     type Error = anyhow::Error;
 
     fn try_from(e: &Entry) -> Result<Self, Self::Error> {
+        if e.attribute.is_empty() {
+            return Err(anyhow!("Attribute cannot be empty."));
+        }
         let base_entry = models::Entry {
             identity: e.address()?.encode()?,
             entity_searchable: match &e.entity {