Fix validator lookup by title.

We want the lookup to yield a result when there's only a single
  validator; and no title is provided. So that users can simply do
  'aiken address' in their project if it's unambiguous. The validator's
  name is only required to disambiguate between multiple validators.

  I also noticed that the order of arguments in with_validator was
  wrong. Somehow.

crates/aiken/src/cmd/blueprint/address.rs

@@ -45,7 +45,7 @@ pub fn exec(
                 validator
                     .as_ref()
                     .map(|v| format!(".{v}"))
-                    .unwrap_or("".to_string())
+                    .unwrap_or_default()
             )
         });
 

crates/aiken/src/cmd/blueprint/apply.rs

@@ -46,7 +46,7 @@ pub fn exec(
                 validator
                     .as_ref()
                     .map(|v| format!(".{v}"))
-                    .unwrap_or("".to_string())
+                    .unwrap_or_default()
             )
         });