decode valueless entires as NaN

since NaN == NULL in SQLite, apparently

src/database/entry.rs

@@ -50,9 +50,11 @@ impl TryFrom<&models::Entry> for Entry {
                 value: EntryValue::Number(value_num),
             })
         } else {
-            Err(anyhow!(
-                "Inconsistent database: Both values of entry are NULL!"
-            ))
+            Ok(Entry {
+                entity: Address::decode(&e.entity)?,
+                attribute: e.attribute.clone(),
+                value: EntryValue::Number(f64::NAN),
+            })
         }
     }
 }